A/c not working but parts all functioning

Not sure what to suggest, you'll have to do the same thing the mechanic wants to do yourself. Get a schematic of the wire harness and a meter and start testing for continuity. You could also disconnect the harness connector and re-seat them. Look for any abrasions to the loom. This is all assuming there aren't any problems with the components and ac control module. (IMO, unless the wore harness looks like it was chewed on by a rat, I wouldn't get too deep into trying to test every pin, instead try testing the module, or swapping it out for a scrap yard replacement.)

I should add I had part of my engine replaced and have had nothing but issues since with different parts. They didnt put a hose on properly that caused issues on something else and resulted in a 400 dollar bill for something that should have been done properly in the first place. Now im so mechanically inclined its not even funny but I have had this car 2 years and already put 5000 plus into it. I clearly bought a lemon but also believe the mechanics screwed with shit on purpose.