The front and rear fan blower guage will only run on high.

Sounds like high blower relay is bad

If I am understanding you correctly, the blower fan is not running. You will need to take things apart until you can get to the blower itself. (Or you could test for voltage at the blower control switch) But I am guessing it is the blower that has gone bad, not the switch. However, they usually get noisy, and then go bad. Test for 12 volts going into the blower motor. If you have voltage at the blower motor, and it doesn't run, replace the blower motor.

The resistor bank is bad for slow speeds, the switch is bad for high speed. In your test, you have voltage, but the switch wont carry the amps, to run high speed. Open circuits will not blow fuses. Replace both switch and resistor bank for all speeds. Make sure to clean leaves and trash from fan housing and see if fan spins freely.

I would start with the rear blower motor resistor, the resistor is what controls the speed of the fan, the resistor is located in the rear heater box near the blower, you will need to remove the right rear side panel to access the heater box. start with that

Test voltage @ blower power wire with grounded test light while key on heater fan on high.
If voltage constant,suspect bad blower motor.

Check the fan relay(or fan module near the blower)or the fan switch itself,or the blower motor is bad,bump the blower motor while the switch,and key are on,if the blower starts working,it is bad.

First make sure that the coolant is full,then make sure that your cooling fan is running when the temp gauge is at high.You may have a temp sensor or relay for the fan that is bad,or maybe the fan is bad.The fan will also run when you have the A/C or heater in the defrost position.If you have duel fans one may only come on with the A/C.Well hope this helps.

The switch or the wire harness at the blower motor has gone bad. The highest setting 5 has no resistor, so the blower runs on high but the high current flow ends up burning up the switch's contact on that setting or the resistor wire harness at the blower motor has gone bad. Common would be the switch it self, if the switch is good. Then it's going to be the wire leads at the speed resistor by the blower motor under the glove box by the passenger leg compartment. Keep me posted, be glad to help you get your Sierra running 100% again soon.

The blower fan resistor has gone bad and needs tobe replaced, You can go to Auto Zone and buy the new blower fan resistor for much less then the dealer and Auto Zone will have the step by step instructions to replacing the blower resistor with common tools, Good luck and hope this helps. keep me posted, be glad to answer any other questions you may have. 

its not in the headliner. its under the side panel in the rear on drivers side. you half to remove the panel on the inside just above the rear tire. since the moter runs sometime i would replace the blower motor resistor first cause these are known for going bad and its in the same area as the blower motor. this resistor controls the fan speeds and if its gone bad the fan dont run right. plus its alot cheaper than blower motor. once you have replaced the resistor you can turn the fan on and see if its a fixya before putting the panel back in place.