I recently fixed a fuel pump problem in my wife's 1993 940 sedan. In the course of this work I checked and bypassed a few relays to eliminate an electrical fault in the fuel pump circuit. Specifically, I flashed out the trouble codes in all 3 modes of the OBD-I, bypassed the fuel pump/fuel injection relay, and the radio suppression relay. When I did this work I do not believe the radio was turned on. I also disconnected the battery to clean the terminals. When I entered the radio code after reconnecting the battery cables the radio works but there is sound only from the front speakers; it's very distorted (tinny) and the volume is low. There is absolutely no sound from the rear speakers even with the fader adjusted to just power the rear speakers.

Apparently, something I did created this problem. Can anyone help me understand the cause of this problem and how I might fix it? The radio is the standard Volvo radio for this car, the CR814.