About 10 years ago I took an AM radio / fader / 8 track player out of a 68 Parisienne convertible.   The wiring was kind of butchered, and I never did figure out how it was to be wired.

Re the tan/brown wire from the multiplexer, on other units with the external multiplexer (1969 & prior) I've seen a blue wire where yours is tan with the same connector, it's for the 2nd audio channel that goes to the rear speaker.    Somewhere I have it written down, but I recall the Left Channel is for the Front speaker, and the Right Channel is for the Rear speaker.

I see the multiplexer has the AC Delco paper label on it, can you get me the model #?   I have AC Delco books for 69-72.

I'm also thinking you shouldn't need an external fader as the stereo radio should have a built in fader for front/back speakers.  You'll be able to tell by looking at the right hand shaft.  Does it have a place further down the shaft for a "Speaker" bezel?  Or is it just round for a "Select" bezel?

Radio light?

Model # 7303362

Tone and selector on right shaft

balance and volume on left

I think the bezels are on the wrong shafts, should be Volume & Tone on the left shaft, Tuning & Speaker on the right shaft.  As the bezels will only fit with an internal fader, your external fader is not required.     I still think the brown/tan wire is for the rear speaker.     I'm not sure if I'll have time to look up your multiplexer in the AC Delco books.