This is a great tip Geddy!

A slightly more elegant way might be to put a dab of solder at the base of the RCA and the metal chassis of the HU.

I have seen noise filters as well. I can't vouch for them but they do exist. Scosche ES004 10-Amp 100-Watt Single Stage Noise Filter at TigerDirect.com

Ghettracer- It's best to ground the RCA's to the HU and then ground the head unit to the body. This is to avoid ground loops.