Last of a three part post on Chevrolet and GMC full sized truck and suv blower issues.
The best place to start testing is at the blower motor. Key on and blower set to high. Check for power on the purple wire and ground on the black wire. If power is missing on the purple wire recheck while switching to reach of the lower speeds.
If no power is found at all on the purple wire, go to the blower relay and check for the signals listed on the part 2 post. If all signals to the blower relay are good except on the dark blue wire, disconnect the blower resistor and check for the proper signals from the blower switch as outlined in the part 2 post.
If power on the red wire or ground on the black wires are missing check the integrity of the three wire connector to the right of the glove box opening.
If the connector is okay but power is still missing on the red wire check the A/C fuse in the underhood fuse box. If the ground signal is missing and the three wire connector is good, repair the loose black ground wire at the rear of the passenger side cylinder head. If you cannot find it in yourself to man up and fix the ground at the rear of the passenger side cylinder head, cut the black wire under the dash that goes through the firewall and attach it to a known good ground.
If you were missing any blower switch signals at the relay and or the resistor, remove the a/c control panel and check the circuits there as outlined in the part 1 post.
Lastly due to age and the type of plastics things may/will break when working with the a/c control panel. The main hold down for the blower switch will hold it in place even if the smaller retainers break. If the latches that hold the control panel to the dash break, the panel can be attached with small drill point screws.