It's screwed into the side of the receiver dryer on the right rear corner of the engine bay. By the evaporator. It has a schrader valve under it so you can replace it without blowing the charge.
Little known about them but they are adjustable. I'm not too comfortable saying how since the OP seems to have a low charge to me.

 

If the ac came on when you would wiggle the plug I would start with checking for corrosion at the plug and clutch. Make sure the wires arent broken at the edge of the plug. then i would check for voltage at that plug-both inside the connector and a few inches back. if there is corrosion inside the wire it is very possible that you will get 12v inside the plug but not enough power to engauge the clutch so I would def use a test light and not a meter to check. If no voltage there then check the plug at the low pressure switch to see it you have 12v. If you unplug it you will only have 12v on 1 wire, if it is plugged in and the lp switch is good you will have 12v on both sides. If you dont have 12v at the plug then it is either a selector switch or a bad wire between lp switch and selector switch

 

okay sorry i got confused what the lp switch was i thought it was the one on the back of the ac compressor. so i will check the one that is actually is tomorrow. is rechargeing the system something you can do yourself?

 

yes you can recharge it yourself its very easy i just did in on my pos sunfire and im having the same problems your are with my truck and i think its low on charge. with 230k on it its very possible. of course wisconsin is the only state where 134a is illegal . so i have to go to illinois to get more freon