Hi!

Chryslers and maybe many other makes have the two left speakers
connected to the left channel of the radio. *GM radios OTOH did or do
connect the front left and rear right speakers to the left channel,
and the others to the right. *

Why do you say this? I don't think it's true--across an 84 GMC Sierra,
88 Buick LeSabre, 89 Buick Electra, 94 Chevy W/T and a 2003 S-10, none
have their audio systems wired this way. Left is left, right is right,
no matter front or back. All have four speakers installed.

I've tested those systems with an FM stereo input signal (from a low
power modulator) consisting of tones that play in either the left or
right channels. I have never noticed them being swapped in any of the
vehicles...playing a sound on the left speakers results in it coming
out of the left speakers, both front and back.

All have their stock speaker wiring, as well as the factory speakers.
Only the 84 GMC and 2003 S-10 have had their radios replaced with
aftermarket units.

As the fader and balance controls produce the expected results on each
vehicle, I doubt that any of the wiring is swapped. From a cursory
look at things, I think that "correcting" the circuit logic of these
controls so that they do the "right thing" with swapped channels
between the front and rear when operated would be complicated.

William