Your problem can range from a dirty connector to not having core components under the hood or dash.
Not much the group can do until you post up some underhood pictures and/or more info, like:
1. Are all the components there? Compressor? Condenser? Accumulator? Pressure switch? Compressor wiring? Ground?
2. Is there charge in the system at all?
3. Does the compressor cycle at all when you have the switch set to AC mode?
4. Does the under dash manifold change modes from hot to cool and from defrost to floor to dash?
5. Is the wiring or selector switch faulty?
Start with big stuff: is the system intact? Don’t laugh, a lot of these trucks dont actually have all their parts.
From there if the system has no charge you probably need to flush and rebuild the seals, then vacuum and recharge.
If it has charge you might get away with recharging it.
If it is charged and the compressor cycles on, then you might have a mixing door problem.
If all else checks out then you need to check electrical connections.
I’m sure I’m missing plenty but you haven’t given much info so you are looking at a fix that can cost anywhere from 50 cents worth of paper towels to wipe off a dirty connector to a thousand dollars or more of parts and labor.
