What I don't have is any voltage at my point of splicing which is indeed the correct wire for reverse lamp illumination. I've tested this with key in "run" position and the lamps do illuminate and extinguish relative to the shift lever.
The entire camera/monitor system is up and working fine, for now I'm simply applying +12 VDC to assure my installation and wiring are correct. Apart from my wanting it to turn on only when "reverse" is engaged all is good for a final install and replacing interior panels.
I "think" (aka assume, guess
) I need to tie an actual tap to the grounding side of the reverse lamp holder/socket. This van had an interior back up alarm that had ceased working but as I recall now it was triggered by the reverse lamps coming on but that lead wire was after the bulb----I'll try replicating that action and all should be (hoping, guessing) that'll work just fine.
I'll update this in case it might benefit someone else at some point.