monsoon stereo problem
 

HI
I have a 2000 camaro ss with the 6 speaker monsoon system. I have a sub woofer installed and upgraded all speakers but the middle bass driver in the back seat. Unit worked great up until last week. Left channel rear doesn't work, left channel door works but only bass no treble. I removed the speakers and they tested fine. My question is the head unit a problem or should i replacethe amp? I unpluuged the rear amp and re-plugged but no luck. I can get a used amp for $50 on ebay or just replace the head unit with a nice pioneer for $200.
thanks
todd