Is the VGA cable for the screen running really close the cig lighter power cables? If nothing is being powered, no interference, but if something is pulling juice, it is making itself a e. field, and causing interferance. It could be a bit of both things here. Something to try if you can is to get the VGA cable from the screen and run it out of the dash temporarily for testing through the dash hole to the laptop away from the power lines of the laptop with it plugged in. If the interferance goes away, then a simple VGA cable re-route will work. But if you power it from a dedicated line, you should also make sure the VGA cable is not routed by the line.