On my in-dash dvd, if it's grounded when the system gets power it locks out as if it was in-motion, you have to then release the e-brake/unground it, and then re-apply to be able to watch it.

There are a few ways depending on your model, the most common way is to get a relay that will ground the brake 10 seconds after receiving power to the unit. Or you could install a switch in-dash to become a 'virtual' brake, when you flip it on, it does the same as hitting the brake.

If you have a nav system as mentioned above, there's a company that sells through amazon based out of Canada that sells bypass kits that include firmware / software manipulations for nav systems, especially the pioneer units.