Have you tried using a new speaker wire? If you have one, just run it from the head unit (or amp if you have one in your system) through the cabin directly to the speaker just for testing purposes. If it works, you have a bad speaker cable. If it doesn't, then you have a different issue, possibly with the head unit or amp itself.
__________________
~Rob 
-DD Air Guide, Stillen G3 CAIs, 63mm Throttle Bodies, Motordyne M370, Berk HFCs, FI TDX CBE (non-res, CF), Sprint booster v3, Ecutek Dyno Tuned, and the list goes on...
|