I just installed a pioneer AVH-P4100DVD about a week ago and my dad and I just bought a simple toggle switch for like $2. I dont know if its the same with every system but we just ran the parking brake wire to one terminal on the switch and then tapped into the ground wire and attached it to the other terminal. Later we cut a hole into a trim piece and mouted the switch. At first it says you cannot watch DVDs as your driving but once you flick the switch it will alow you to watch them =]
I hope this works with your system also!