Well, that didn't work out like I wish it had. I bought a used radio off e-bay and it arrived just now. I took it out and installed it, and Voila: I still only have one speaker that works--the one in the passenger-side door. Worse yet, I put a CD in the CD player and it's not playing, and in fact, the player won't spit the CD back out.
Sounds to me like it's a wiring problem, although one would wonder why 7 out of 8 speakers don't work...but one does. On the UP side, that speaker sounds a lot better than it used to with the old radio. But for $90, that's not a very bright side.