To address your unanswered questions, I dont see anything wrong with using a usb extension, I do it all the time on other units. The extra OEM plug that doesnt get plugged into anything is for the factory changer/aux in/ipod interface, none of these are used with the dynavin so just push that plug aside.
All of your assumptions about the unused wires are correct, if you arent using them, just tape them up, and splice the parking brake wire into the black ground wire and you are good to go.
Oh and to correct one thing for future reference, the reverse wire isnt to power a reverse camera, it is to switch the unit into reverse view, to see the reverse camera, so it is spliced into a wire that turns + when the car is put into reverse (like the reverse light circuit). You wont be using this.
....get that thing installed at let us know what you think!
Last edited by jeffb325; 02-27-2010 at 10:22 AM.