Not that this could be an issue but I have seen the wires come loose inside the connection. I’ve seen the moulded connections not making a good connection inside the moulded part. I’ve cut the moulded plastic off and found the wires loose. I generally use studs in the side post batteries with nuts as screwing the bolts in/out eventually will strip the threads. Check the grounds not just the 1 from the battery but the 1 from the engine to the frame, firewall to the frame and to the fender by the battery. The engine harness as well. All these trucks are old and the wiring gets fatigued from age and heat and cold. If the throttle body has never been cleaned where the IAC seats can get carboned up and prevent it from seating correctly. Not that any of this your problem but stuff I’ve came across. Please post what you find the problem as it very well could help someone else having the same issue. Electronics are get when they work but when they don’t oh boy and now you have to add in cheaply made parts and covid parts on top of it all