[EDIT] Dang it !!!! djmlambert, you beat me to it and posted while I was still typing..
. The blower relay controls only the hi speed. The relay has a separate "hot wire" coming from the fuse box that is hot all the time. The relay receives a hi speed signal from the blower speed control switch through an 18 ga. orange wire. The hot wire for the relay is a 12 ga. also orange wire.. The relay will "click" when it receives the signal through the 18 ga. orange wire, but if the 12 ga. orange wire has no voltage on it, the blower will not work.. Check for voltage on the 12 gauge orange wire at the relay..
A complete AC electrical system wiring diagram can be found in the Heater/air conditioner section of the GM service manual for these trucks..