Take the radio out of the dash, put a meter on the harness, there will be a couple of positive 12v leads, test each one with the car and park/drive, see if there is any fluctuation. Confirm that the ground connection stays when the shifter is moved.
Turn the power to the car off, and check continuity of the speaker wires to ground.
Let me know what that gets you.
I'm assuming you have a multimeter, if you don't, i'm sorry.