I really am at the end of my knowledge. My setup:

Ford XR Australian made car with built in color screen (for displaying CD's, heater controls etc) and a HP laptop. I am interfacing to it via a dsub cable and custom cable at the other end. I use powerstrip to hack the resolution and frequencies etc.

The setup is perfect when the laptop is being powered by 240v mains OR laptop is running on battery. The issue is I cant use the car to power it. When I use a car DC laptop charger the screen flickers shockingly (power filter didn't help). I then tried a cheap inverter and still shocking. The best effort was my 3rd attempt which is an expensive sine wave inverter and it works awesome compared to the other two methods.

However, there is still flickering. If I ground the inverter its real bad but with no ground its getting close to acceptable. I have spent a fortune on this project in money and time I really need some help. My guess is that this is the video version of ground loop?