i have an annoying noise problem,this is my setup

p4 2.4 running off 300w softstart inverter
logitech z680,s running off separate s\s 300w invertor
Cartft 7" touchscreen running off cigerette lighter psu with "egg"

This setup works absolutly fine, ie perfect sound and picture crank survival etc.

If i use my minidisc player in "line in" on control center it also sounds fine.

Now for my problem, i linked my car cd player to the z680,s through my speaker wires through a spkr output\line out convertor into the line in on the control center, this works fine without the engine running but with the engine running i get high pitched noise that gets worse the higher the throttle.

If i disconnect the car speakers the noise disappears,but i like having the speakers connected and want to be able to play cds without the carpc on, the car stereo itself dosent have this noise.
Both the inverters are earthed and have noise filters on the DC inputs.
The Z680,s and the pc are connected to the inverters earth via the plugs.

Any thoughts would be appreciated