As mentioned before, you CAN watch DVDs and Nav input without the parking brake being on - you just have to be going zero MPH.
The stereo listens to NAV and/or speedo to determine if you're moving.
The Lockpick C8 intercepts this signal and tells the stereo 0mph. While the lockpick is active, your Nav system always will show '0mph' but time-to-destination and whatnot still works.
Oh, as for cheaper, they make the Lockpick C8r - which is a bit cheaper but doesn't have the video-in option for additional cameras.