-the a/c compressor relay would be located in the fuse box under the hood on the driver's side. goes for around 10bucks. remove the relay from its mounting and test it on a bench. Use an ohmmeter to test for continuity between the relay coil terminals. If the meter indicates an infinite reading, replace the relay. If there is continuity, use a pair of jumper wires to energize the relay. Check for continuity through the relay contacts. If there is an infinite reading, the relay is faulty. If there is continuity, the relay is good andyou need to test othercircuits.
-the ac cycling switch should be under the low pressure port and is hand tight, I know on some models this can be removed without discharging the system !!!!I'M NOT POSITIVE ABOUT THAT ON YOUR TRUCK!!!!goes for around 10bucks also.<--- further, i don't know how to test that switch for functionality or if its even possible. maybe someone else does.
-there is a switch under the dash but i'd not bet on this as the problem.
the symptoms don't really relate buti'm not counting it out, however,i'd put my money on the compressor relay. pressure switches aren't known to go bad so much as relays do but they can and do. but you'd also be left to wonder why the relay went bad... but if your pressure sensor was malfunctioning i'd think it most likely to shut the compressor off and leave it off. after all this is just a suggestion. i'd check the relay first.