I think i see your problem.
ALL/MOST monitors have a parking brake feature on them.
Now dependign how you have wired this up, it will only allow you to watch video/or enter setup while your handbrake is on.
Now if you wanted to use the unit all the time. You have to get around this problem
*i take no responsibility fro you doing this, especially if you crash!!*
Normally, all it is is a wire checking there is a ground, connect the parking brake connector, (on bak of HU) to a permanant grnd.
Then u shud eb sorted.