then it's a matter of trouble shooting.....could be anything.....could be the ground itself, the rca's and so on.....no easy fix unfortunately.....just start at the front with each component and work ure way back.....a bit of work but worth it in the end.
oh, and just a thought....if ure running ure rca's down the passenger side....make sure they don't run directly around or over the ecu.....that can be a source as well.....