Head Unit (radio). You should have seen it when you tested the other leads. Also, check to make sure the ground is solid and that the constant power is connected well.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>