Blower motor

The heater core in a GMC is always "on," meaning you always have engine coolant flowing through it. If you have heat at any blower setting, then the core itself is working, but it wouldn't hurt to check coolant levels, as low coolant can cause erratic heater core operation. Some vehicles do have a heater control valve, that controls flow of coolant to the heater core, but your vehicle does not have that. Your heater core should be hot all the time, assuming engine is reaching normal operating temperatures. Try touching your heater hoses to see if they feel pressurized and hot/warm to the touch. If not, then there is no flow to your heater core. You'll have to troubleshoot to see if that is caused by your heater core itself, or a problem with coolant flow caused by a faulty radiator or water pump. As to the the blower motor, if it only works on it's highest setting, then you may have a problem with the blower motor resistor pack, which limits voltage to the blower motor for all lower settings (at high, the resistor pack is bypassed so full voltage reaches the motor). If blower motor is cutting out at high, you'll have to troubleshoot, problem may lie in wire harness or switch. Lastly, wouldn't hurt to check that you don't have a problem with a blend or mode door operation, the blend door needs to be open to the heater core for you to get heat to the cabin. Blend and mode doors are moved by an actuator which receives voltage from your selector switches.

Boy, you have that covered there KenF. Like KenF says, replace the blower motor resistor.

Blower was not blowing heat...come to find out it was a fuse... works great on 3 and 4 bur not on 1 and 2...wife is happy we have heat