My daily driver ('06 RAV4) never had a backup camera as well and I bought this camera to work with the same Pioneer HU. I had to make a new hole by the license plate to mount it, route the cable through the back door grommet, and connect the 12V reverse and ground wires at the taillight. The included cable was long enough to run down the door edges to behind the dash where the RCA connector plugged into back of the HU, there was also a red wire that carried the 12V reverse signal that gets tied to the violet/white wire of the Pioneer harness. I took out the back door and lower passenger quarter panels to run all the cable and tap the taillight wires.
For your '08, it will be similar where you will need to remove the lift gate panel, mount the camera, and route the cable through one of the grommets. If you want to tap the 12V reverse and ground wires at the taillight, it will be more involved to take the taillight out (dropping the rear bumper enough to remove one bolt), but you can run the cable down the door edges to the dash. Otherwise, you could run the cable through the headliner and down the a-pillar, but you'll need to find the 12V reverse signal in the front somewhere. It may depend on the 12V/GND wire tap lengths since they're typically not long enough to reach the front of the vehicle unless you extend them. The Natika was that way, so I had tapped it at the taillight.
For your '08, it will be similar where you will need to remove the lift gate panel, mount the camera, and route the cable through one of the grommets. If you want to tap the 12V reverse and ground wires at the taillight, it will be more involved to take the taillight out (dropping the rear bumper enough to remove one bolt), but you can run the cable down the door edges to the dash. Otherwise, you could run the cable through the headliner and down the a-pillar, but you'll need to find the 12V reverse signal in the front somewhere. It may depend on the 12V/GND wire tap lengths since they're typically not long enough to reach the front of the vehicle unless you extend them. The Natika was that way, so I had tapped it at the taillight.