Obviously, a screen playing a dvd within view of the driver poses a real safety risk, so the manufacturers apply certain safeguards, in this case forcing you to wire your TV to the parking break in order to view the screen. When wired to the parking break, this requires you to be stationary and have the parking brake engaged in order to view the television.
When wiring the dvd unit, you can bypass this safety feature on some models, while others require you to buy additional parts to accomplish this. Most newer models have switched to a system that senses motion and won’t turn on regardless of how you wire it. However, some are still possible to bypass the old-fashioned way.
Pull the unit from your dash (assuming it is installed already).