Hey Joe. I tapped into the reverse wire for the headunit at J33, which is over by the engine control module. I actually ran accessory power to the camera so I can switch to it whenever I want - not just when in reverse.
The wire going to J33 comes off of the park/neutral position switch, to J33, then on to a junction connector (J40), then to each back-up lamp. J40 is in back just forward of the bottlejack (under the trim). Here's a a pic of J33: