hi, i had the same problem in my pulsar, i added all the off the shelf filters and stuff and it still was there, i tracked it down eventually to poor rca leads, which was the last thing i thought it could be but it turned out that the rca leads that came with the wiring kit were not properly shielded, that is the shield did nothing, it is supposed to connect from one device to another, i went out and bought a high quality set of rca leads and that ultimately fixed the problem. BEWARE OF DODGY RCA LEADS, EVEN IF THEY LOOK COOL.
The best investment any car enthusiest can make is a good quality multimeter, like from altronix or jaycar, mine cost me 10 bucks but potentially saved me hundreds by not having to take the car to a auto elec.
another thing you can try is getting a ferrite core from dick smith or jaycar etc and completing as many turns as possible of the speaker wire around it, this will eliminate most RF interference by not passing it into the speaker cable but allowing audio frequencies to pass.
dave.