Make sure All Signal Wires (RCA's) are away from any power wires or antennas, Inducted noise from outside source is very common for cars of such "Quality" lol. But seriously MECP standard is that any power carrying wire should not run along or next to any signal carrying wire. Ex: Run power on the driver side and Run Signal on the Pass. if they should pass Make sure its at a 90 degree angle or no less than 85 degrees.

Sounds like you have a Dirty Signal to me, Are you using a Line output Converter? or do you have any after market headunit and using the preouts on the headunit?