Dick Benjamin's Multiplex Stereo Installation on his 1968 Imperial

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I have finally, after 14 years of looking, found all the components to the complete 5 speaker multiplex stereo sound system that was originally installed in my 1968 Imperial Crown 4 DR Hardtop. The original owner, during his last few months on this earth in 1986, took the car to a radio service facility in Southern California for repair, and didn't keep any record of where he had taken it. Those folks removed the whole system for testing, excepting only the speakers.

I had known of and admired the car for many years; about 6 months after the owner died, I asked the widow about it, and she agreed I could buy it. Unfortunately, there was an obvious hole in the dash, and no information as to where the "radio" was.

I did not realize this at that time, but since the car had the 5 speaker setup, it was obvious that I was missing a lot more than just the "radio". In addition to the 5 speakers, there are a crossover box, a multiplex signal processor, a stereo fader control, and of course the AM-FM Stereo radio. There are also roughly 15 electrical cables, each with a multi-pin connector on each end, each connector holding up to 9 wires. This system is extremely complicated, to put it mildly. Such was the state of the art in 1968.

After many years of haunting parts dealers and swap meets, I have pieced together a complete system. Unfortunately, the Factory Service Manual does not really cover this system, in fact the wiring diagram completely ignores the main multiplex converter unit that is mounted behind the rear seat. I found that the information that is present in the wiring diagrams is incorrect in a couple of areas, and just omits a few of the important connectors and cables!

Last week, I began to try to reinstall the system in the car, and discovered that I did not have all the cables and connectors, and that I did not even have a clue as to which wire goes where in the system. Accordingly, I set the system up on my test bench, and with my oscilloscope, test signal generators and meters, figured out all the wire connections, and recreated the missing cables. You can probably imagine my delight when I fired it up for the first time, using 5 makeshift speakers, and a jury rigged antenna. The deep, rich bass tones and the realism of Stereo was such a treat, that I just sat there listening to the music for a while, before removing the seats and dash innards to reinstall the system in the car. It was a lot of work, but well worth it.


The 8 track player always was installed with the 5 speaker sound system, including the crossover network. However, the Multiplex Stereo receiver was not a mandatory radio for the 8 track system, any of the radios might be present in a car with the 8 track player, but of course if the radio is not the Stereo Receiver, there will be only monophonic sound from the speakers. The Stereo Receiver requires the Multiplex Decoder to operate. The cables that connect all the units and the speakers are very complex, and anyone contemplating making one of these installations must be certain to get all the harnesses that come with the system.  The sound from this system is superb, every bit as good as a modern stereo setup!

Here are some photos of the specific component parts of the Multiplex system.  Please click on any small image to see a larger, more detailed picture.

Model 383 AM-FM-Stereo Receiver. Compare the face plate to the much more common Model 419 AM-FM Mono "Search Tune" receiver. Note that the Search Tune buttons are replaced with a black "Stereo" indicator, which has a light to indicate reception of a Stereo Signal. The Stereo Receiver does not have the Search Tune feature.

Model 491 AM-FM signal seeking radio. Compare the face plate to the Model 383, note the Search Tune buttons.

Model 391 8 Track Player, mounts under center of dash.

Part No. 2864 724 Multiplex Decoder unit, mounted behind rear seat brace.

Speaker signal splitter for 5 speaker Stereo sound distribution, mounted to glove box rear.

8 Track player unit (before refinishing).

There are three front speakers, one on each front door and one in the center of the dash.

This shows the mounting of the speaker to the door panel.

This is where all the hardware has to be stuffed!




Here are some photos of how the installed Multiplex system looks.  Please click on any small image to see a larger, more detailed picture.

Here's how the system looks with a tape playing. Control is automatically transferred to the tape player unit. The Receiver is not being used, except for the speaker system.

Kenny Rogers tape playing, control is by volume, balance and tone controls on the player itself.

Showing the Stereo Receiver in use, no tape in tape player.

Showing the Stereo Reception Indicator light in operation.  "Rhapsody In Blue" coming out of 5 speakers at 120 dB!



Here are some questions and answers for Dick about his installation:

Question from Andy:

Where are the 5 speakers located? This topic came up on another list I'm on (not about Imperials) and everyone seemed to agree that a fifth speaker would not sound right on these old two channel stereo setups. I'm curious how Chrysler did it on the Imperial.

Also, do you remember the name of the manufacturer that built the unit, and how does the sound compare to newer systems (in your opinion).

I'm curious, because I have a 1969 Thunderbird with a factory Bendix AM-FM stereo, and it's all in one unit. It has four speakers, two in back and one in each front door armrest. I have replaced the speakers, and it sounds great. I like it as well as the Bose system in my daily driver.

I looked at a 1968 Imperial LeBaron a few years ago that had AM-FM in it, although I don't recall if it was the stereo unit. It probably was, as I remember thinking it sounded really good for a car of this vintage.

Answer from Dick:

The standard AM-FM also has a very good tone, with rich bass and excellent sensitivity. I have those in my other 3 boxcars, and a spare one in my shop on a battery eliminator for daily entertainment.

The AM-FM with foot operated search bar was by far the most common radio in 1967 and 1968 Imperials.

The AM-FM with Stereo Multiplex was slightly different in appearance, and did not use the search tune feature. The location for the search tune bars above the dial was occupied by the stereo indicator light, and the floor button was not used.

The 5 speakers are located as follows: front right and left speakers are in the front doors, with a square black speaker housing which slightly protrudes from the door panel, just above the lower trim panel, toward the front of each door. These could be mistaken for aftermarket add-ons, but they are factory. The front center speaker is in the normal "old time" location, and the rear speakers are quite large, and mounted at either end of the package shelf.

The purpose of the front speaker is to avoid the "hole in the middle" effect that was somewhat disconcerting on most early stereo installations. Each of the 5 speakers is driven by a separate audio channel, so this front speaker was definitely not an oversight.

The 8 track unit is suspended under the dash, with a bronze panel to match the rest of the interior trim.

This installation is a one year only setup, as by 1969, they had redesigned the units to incorporate all the electronics in only two packages.

The sound is exceptionally clean and pure, with very rich bass and a pronounced stereo effect. The power output is not up to modern standards, so if you are in the habit of taking your Honda to a picnic and leaving the doors and trunk open to entertain the whole 30 acre park, this is not the car for you. But for anyone with normal hearing, the car produces as close to a concert hall experience inside as any car I own, and that includes two current luxury models.

I am not certain who the manufacturer was of these devices, but I would guess Delco, as they were in the forefront of Stereo Radio development for autos in those years.

Question from Bob:

Can you tell from the wiring if there was any attempt to create a "pseudo 4 channel"? This would be just a short time before that fad hit and one method to accomplish this was an out-of-phase wiring of the rear speakers, the details of which escape me now.

Answer from Dick:

The speaker wiring is quite definitely not set up to do the phase reversal trick, as the (-) wires for all 3 front speakers are tied together. The rear speakers are driven by 4 independent wires, so it is possible they were playing some sort of game there, but the sound is quite obviously real stereo, and very nice too! There are two distinct audio drivers, one for each channel, and the front center speaker probably is run off some mix of the two channels, I didn't check. It is really a two channel system however, not 4 channel, which would require different source signal (broadcast format).

I am not aware of any 4 channel systems other than in theaters or very upscale home systems, as that would require a special recording format.

Question from Andy:

I thought the center front speaker would throw off the sound somehow. Does anyone have photos of the factory tape player? I don't think I've ever seen one.

When did the Imperial get the in-dash AM/FM/tape combination, and did the Imperials always have 5 speakers?

Answer from Dick:

Only the Imperials with stereo had 5 speakers, at least in 1968. Stereo did not mean a stereo radio, necessarily. One could order the 8 track stereo player with the plain AM-FM radio, which was apparently the way my car was shipped from the factory. Later, a stereo radio was installed also.

Question from Roy:

Geez, and I thought that ATC involved a lot of hardware! Kind of tacky that they didn't blank out the hole in the carpet where the floor button is for the regular radios.

Answer from Dick:

This floor button confuses me also. I doubt they would have shipped the car that way.

I think this car may have been originally delivered with the multiplex 8 track, but the standard radio - that was a somewhat cheaper way to get stereo, although it would of course be stereo only when you were playing a tape. I do know that the 5 speakers and the tape unit were in it from new. The widow remembers that the radio was also stereo, but I take that with a grain of salt. Of course it could have been changed when the car was new, as was done with the paint (he ordered Gold, and got the cream color, and made the dealer repaint it!)

This page was last updated on February 4, 2002. Send us your feedback, and come join the Imperial Mailing List - Online Car Club