I assume its for the Clarion unit?
If so, then just wiring the parking brake wire to earth (like most other brands) wont work for your head-unit. What you need to do is wire a small switch inline with the parking brake wire and earth, then when you need to watch movies while driving etc, just give the switch a flick on and it will work fine.
What i usually do is use a switch with a "momentary" action. So basically the wire is earthed at all times, but when you flick the switch the contact is broken for a second or so......once you let go of the switch it returns back to being earthed.
Dont ask me why this has to be done.....i suppose its widely known that all you need to do to bypass the park brake function is earth the wire, so Clarion decided to make things a little harder
If by any chance its not a Clarion unit, then earthing the wire permanently will work fine. If youre after a way to bypass the handbrake when driving, then just wire up the same switch to earth, with the output going to the handbrake wire thats connected to the handbrake light, if you get what i mean.....i can always draw up a diagram
Anyway, all this information is not legal and by all means i dont recommend it :whistling