Stop making this so hard.
Here is the "what it is" concerning the factory sound system.
1) The head (AM/FM/CD player) is a two channel device. Its horrible, it should have never been put in the car and those who built it were borderline retarded.
2) The amplifier is a four channel amp. It's not a bad little amp, but not really high performance.
3) Four wires run from the head to the amp.
....... 2 wires for the left channel
....... 2 wires for the right channe.
At the amp, each set of wires are split into four wires, so the head's left channel drives two channels in the amp:
1A) Left door speakers
1B) Left rear speaker
And the right channel drives to channels in the amp:
2A) Right door speakers
2B) Right rear speaker
This thread explains it all:
https://www.crossfireforum.org/forum...-channels.html
Unless you buy a working used Crossfire amp, buying ANY amp means some re-wiring. I suspect your door speakers (both of them on that side) are OK, but your rear speaker has died due to age. They can be re-strung, tho I've never tried it. As to why the OTHER side does not work, could be the amp or the head.
You can get NICE audio out of these cars by doing this:
Replace AM/FM head with something modern.
Run four extra wires to amp (so you can have four channel sound).
Replace the factory amp with a modern four channel amp.
Replace rear speakers - take out those damn dual-coil things and go with something more suitable and common.
Cost will be around $500-$1000 depending on how 'nuts' you go. My JVC head was $85, there are some nice amps for less than $100, speakers can be $10 to $150 each and labor will run at least $200. Can you fix what you have? Probably. Lift the mat up on the passenger side - is there water damage to the amp? If there is NOT, you probably have a bad speaker or two and maybe the head is bad. If you see water damage to the amp, I"d just start over with a whole new head and amp - forget the old wiring, water damaged connectors and pins are a waste of time - go to a sound shop and tell them to start over.