i had the same problem with my isuzu and after i wrapped the rca and speaker wires with electrical tape it was all gone! If the power wire crosses the rca at any point wrap there as well.
like i said, it worked for me might not for you. worth a shot.
1998 SE ATX Dual exhaust Body conversion Nakamichi CD-40z HU Sound by Crystal Mobile Powered by a/d/s M860 AMP My Ride