I have no idea about going thru a head unit, but the LCD display (screen) I have on my wireless system is not powered thru the camera, it recieves it's power from the wiring harness I tapped into. The transmitter recieves it's power from another source, (I tapped it into the aux wire) and then the xmitter sends power to the camera and of course the camera sends the image back to the transmitter which sends it to the LCD. I have toggle switches to power up or down the LCD and transmitter seperately.
There should of been a set of instructions with the camera kit with a very simple wiring diagram. Most screens are set up to be plugged into the cigarette lighter and the xmitter spliced into the reverse lights so it only recieves power when in reverse. (mine is set up to see the front tires, so I did do some modifying)
The head unit is where I am lost, I know nothing about them.
Hope I didn't cause more confusion.