I have one of there, here is how i did it:
extra parts needed: more wire and a switch that has 3 connections on it, the top and middle are connected one way...bottom and middle connected the other.
I first went through and wired it like you are supposed to, to the parking brake
-then I took the wire that comes from the deck, cut that, and hooked that up to the middle prong on the switch.
-then on one side of the switch you wire it to the brake
-other side you wire it to groung.
the reason for this is because the unit needs to see ground/nothing/ground.
you just flick the switch up and down once and there you go...video when you want it,,but if you want to be safe, hit the switch one way, and it works as intended...with the parking brake.
hope that helps.
the switch I got is like $3 at Radioshack.