Tapping the reversing wire at rear?

OK, I know there's a chance this may have been addressed somewhere in another thread but I haven't found it yet.

I'm installing a reversing sensor in my 3rd gen jap import 2009 Prius (right hand drive of course).
I'm just wondering where the best location to tap the reversing wire is as the ECU will be installed somewhere beneath (or to the side of) the storage compartment in the boot. Has anyone else tapped the reverse wire from the rear? If so where is the best place to locate this. Obviously I can access the rear of the reversing bulb via the small pull out panel but there is very little exposed wiring to tap there and I guess there must be a better place to do this. It's winter here in Wellington at the moment and unfortunately I don't have the luxury of a garage to do this in, so any time saving advice will be much appreciated!!