you can add an adapter the the stock unit that will take A/V in. Camaras are as little as a couple hundred bucks. You are prolyl best off just getting a turn-key kit for a little bit more. The install is nada and ya dont have to tap into the stock unit. You could put the monitor wherever ya like... sunvisor, ect.
I ustilize an after market deck as the monitor for this. works awesome