I have a unique problem and am wondering if anyone has heard of this happening and possibly and easy fix.

I previously had the same setup in a different vehicle and had no issues at all. My problem now is when I plug an Ipod or any mp3 player to the input cable that plugs into the back of my headunit the sound only comes out of the passenger side speakers. I know i don't have speaker wiring issues because CD and radio work just fine out of all speakers. Just auxiliary input causes issues.

Any suggestions? Thanks.