Savedmoss

I'm looking for some suggestions on how to resolve an issue with the rear speakers in my 93 GS. This car has always been in my family and has just recently given to me. My mother had an aftermarket radio installed a couple years back and it seems that the rear door speakers do not work. I know they worked fine with the stock radio, but I'm not sure if they stopped working when the radio was installed or sometime after. The subwoofer does work as do all the front speakers. Any suggestions on where to begin with this issues?