do your RCA cables run the same path as your power cables? If so, you need to split them apart, or buy better shielded RCA cables.
Otherwise - I think there may be filters you can put on to take the whine out. I don't have any experience with those personally...
Edit: Something like this, if rerouting the wires isn't an option. Generally try to keep audio wires on one side of the car, and power on the other side.
http://www.lesscoelectronics.com/GLI...ject_p/gli.htm