Lots of good input (except maybe the flashing part and the closing of curtains - you won't know what you are missing).
I obtained the GM P32/42 Chassis manual and although it doesn't clearly specify, there does appear to be two wires that come out of the turn signal switch at the column in the schematics. That will be where I start and then if RearView Safety returns my message I will know if they have a built in delay so that it I will be able to connect directly to the signal side (after the thermal flasher). In their install manual it simply says to connect to the rt and lt wires with the yellow and the white, respectively.