it would be nice to have a few pictures so we can help you, but ill try now anyways

the vx is a limited production car but the audio isnt, its basically the same old same old in every isuzu form 99+. walmart or any audio shops probably carry the harness and trust me it makes your life a lot easier (i spliced the wires.....not fun).

have u removed the cd changer yet? if not then i believe that is the changers harness, completely useless if you are using a aftermarket deck.

as for the parking brake wire, it is located in the center console called, i have no idea what color the wire is and right now i am too lazy to look it up.

ps. generally if you connect that wire to the parking brake light it allows u to turn on the screen for dvds or access ur rca video channels. so if thats the case on ur deck then u wont be able to use it as a rear view camera monitor.