As a general rule, noise in a car stereo is something not grounded well

There are a few things you can do:


put a loop in your coil wire (old trick, worth a try)

wrap your audio signal (RCA) cables in aluminum foil and secure with zip ties (ugly but does work)

buy new high quality RCA cables

ensure your signal and power cables do not run parellel to each other at all anywhere in the car

if your RCA cables have a small wire in the middle of them, I ensure you that is not a remote turn on wire, that is a ground wire. Ground the small wire to the chassis of your radio and to the other end to the body of the car.