My dad had a GMC2500 work van. Retired the 5.3/OEM transmission without ever doing work to either, beyond a water pump or three, after 43x K miles. They run real long time if you only love them a little.
Replaced the passenger mirror assembly today and I saw some "interesting" wiring behind the door panel. Looks like it is all related to the aftermarket radio. I will go back in at some point and fix it, but I had to laugh at the mess.
That’s for an aftermarket alarm - the diodes and relays are a dead giveaway. You can’t interface directly to GM door locks, they are positive trigger I think.