Thanks for the tips.
The car PC is running off a PSU I built myself, not an inverter. The CarPC is grounded (including the case) to the 12mm bolt holding in the seat using thicker cable than the power cable (just in-case), HOWEVER, I havent filed away any paint, ill check that tonight.
My car is a 1992 model, and I cant seem to find a 'C' size battery looking thing anywhere.
I did a small test on the setup I have and the noise seems to be coming through the power cables, not the sound cables. Ill make sure everything is grounded properly tonight and let you know how it went.
Thanks for the advice