[notout86]

Sorry, by THAT pin 13 I meant on socket CN6 28pin

I've gone over your diagram and think I have a good understanding, but here are a couple of questions:

on the SPDT lower r/h corner is "30" ... what is that? .. I found the label conventions for 85,86,87 & 87a and,
if using an RCA-Y to pin 8 of CN4800 and only one cam is powered at a time from the DPDT relay to pin 7, then is switching of video in the DPDT needed as video would only come from the powered cam?

Our car has seat warmers turned on by two neat 3-way rocker? switches on the console near the gear lever. I think one would lend itself nicely to camera switching by pushing down the forward part for the front cam, leaving off for rear-cam-by-reverse gear and pushing down the rear for rear-cam-on-anytime. It is not so cold here that we even think to turn on the warmers and I wouldn't miss mine.
I see you have a Mustang. My wife needs a bucket of water thrown over her whenever she sees one, by which I mean she gets all hot.