1990 740 gl radio power but no sound

[Deeseven]

Hi there, I just got done replacing the heater core on my 1990 740 gl. I am in the process of putting the dash back together. When testing my radio (stock radio and amp) I learned that yes it gets power (turns on, I can hear it spinning the cassette...) but the only sound I get is at full volume and even then it is barely a whisper. I have checked fuses, opened up the amp (located near the steering wheel) checked wires (everything appears to be plugged in). What else could it be? Thanks for reading if you got this far.

P.S. are there any fuses or relays I might have missed?