Need to isolate what is causing the problem: radio, amps,. etc. 15 years ago a filter may have helped but today most radios have adequate filtering. Especially Pioneer, they make solid radios. Get a boom box that has a RCA input on it. Mke sure you use AC power or batteries to power the boom box. By using the boom box as the amplifier you break the reference to ground. It allows you to pinpoint the problem. Connect the output of the radio to the input on the boom box. Get a pair of headphones. Make sure the car radio volume is all the way down. Turn the volume up all the way on the boom box, turn the car on, put on the headphones and listen. If the noise is being caused by the radio or RCA wires it will be present through the boom box. If the noise is present, then run an exta pair of RCA's outside the car from the radio to the boom box. Still have noise? If you do then it is the radio, or the radio is picking up EMR. If the noise goes away, the RCA's are picking up EMR and must be moved. Let us know what you find.
If you determine that the noise is in the radio, try disconnecting the FM antenna lead. Sometimes you can get a ground loop from that.