Just a guess, but if any part of the system is working - i.e the tweeters, it seems most likely to be worn or damaged wiring rather than the head unit itself. so I'd look at that first. It could be as simple as one or more plugs coming loose. Systematically work out from there
I agree on the wiring being most likely. The smaller door speaker shares the feed from the amp with the tweeter (splits off in the front footwells) so if the tweeter works but that small door speaker doesn't then check there (and connections on the speakers themselves). For the intermittent issue, and the rest of the speakers/subwoofers not working, check the connections to the amp in the back and in the doors. Of course for the subwoofers you also have the volume/level control up-front to check.
Did it stop working suddenly, or has been an issue for some time? Is it the original factory system?
I had a similar problem with my 93 Softdash. Original head unit had packed up so I replaced that with a modern unit but the sound through the speakers was either very distorted or nonexistent.
Took the original speakers out and had them tested and although they looked ok, none of them actually worked properly. Replaced them with modern equivalents including the tweeters and now have a good, clear sound system.
If your wiring is original and unmodified then I’d put money on the speakers being the problem.
I also have been trouble shooting the stereo in my '91 SWB. I've replaced the speakers as they were shot, but am having problems with the sub and one of the rear speakers. I believe it is the rear amp and I was wondering if I can replace the amp & separate crossover with a single 4 channel amp and use two of the channels to power the dual voice coil sub? I was thinking this unit https://www.crutchfield.com/p_206PXA3004/Kicker-42PXA3004.html - I really would like to keep my original head unit, but I'm definitely a noob at the car stereo stuff so any advice would be appreciated. Thanks!