Sounds like the touchscreen is getting noise coupled into it from the ground of the car being different from the touchscreen's power supply. Assuming that they

Sounds like the touchscreen is getting noise coupled into it from the ground of the car being different from the touchscreen's power supply. Assuming that they have something like 12V avail to power that unit, then a buck that gives you 3.3V, so probably somewhere in that chain there is a bunch of noise coupling in.. common mode noise.. gotta squash that..