So that's definitely a power problem. Sounds like it could be one of 2 problems: input voltage and/or output voltage too low. For input I mean, if the car battery is fairly drained and not providing enough voltage to the regulator then you'll have a low voltage output on the regulator. Similarly if the regulator is not properly rated for the screen, even if the battery voltage is ok, you have the same problem. When this happened to me before, the screen would initially show some life, but there would not be enough power to keep the lcd powered. I troubleshooted this using a multimeter and off I went.
It makes sense that this may be a car battery problem if your MB won't post. The power for the screen is not drawn from the MB, as the usb is only for the touchscreen controller. If input voltage is off then your MB PSU wouldn't provide enough power.
A multimeter is your best friend for your problem. Use that to test voltages at the various points and you'll be good.