well usually interference is caused by rac to close to power cables

i done a system in my old escort and the only time the power cable was near a rca was when it was going into the amps, they were on the opposite side of the car

i fitted a pioneer 6200 which was new a couple of yrs back and it hada major interence, so i put supressors in the rca and still had slight interference so had each amp have hits on earth and all was sorted

what i realised was it was the headunit causing the interference, i put another head unit in and it was fine with no supressors



just a thought of try a different headunit if you have one