OK, to revive this thread, I bought Colzie's old Pioneer DEH-P4650MP head unit, and there's noise feeding back through the RCAs to the head unit. Is there any way I can fix the problem with the poor ground tracks in the head unit? Will soldering a few ground wires between the RCAs and the preamp in the head unit help?
Everything's grounded properly, yesterday arvo I ran a 10ga ground cable from the head unit to the dash and also to the amp ground, and the head unit and the amp are running off the same power wiring using a junction block under the dash. I got an RCA isolation transformer from Strathfield, but I realised once I got home that it only had a frequency response of 40Hz-34ish kHz, too high for an amp running a sub. It did cut out most of the noise but it also cut out most of the bass.
I just checked the rating on the Jaycar isolation transformer, it's 20Hz-20kHz, I'll go and get one of them once they get more stock in.