The only parts common to both systems are the ignition switch, power distribution center and the HVAC control module. As you did not state other problems like lights/stereo/other systems we can rule out the ignition switch as it also controls the power to all the rest.
Next the power distribution center also could be an issue, however it is very unlikely.
So let's start with checking the fuses, both items are high power draw so the fuse contacts in the panel could be failing. Unplug them and look to see if the contacts on the fuse or in the box are a different color (usually they will be silver, if they are failing they turn grey, blue or black)
Fuses in the under-hood fuse block.
Fuse 4 Batt Main 2 (60 A) - feeds the rear defroster
Fuse 3 Headlamps (60 A) - also feeds the heater blower.
Fuse 29 HVAC/RKE (10 A) - feeds the HVAC module
Instrument panel fuse block
R-Defog fuse (30 A)
DIC/HVAC fuse (10 A)
HVAC CTRL fuse (20 A)
HVAC HI fuse (30 A)
If none of these are blown the likely culprit is the HVAC control module. They are a problem issue on many GM vehicles. Some are repairable if you know how to solder and can find the correct info online. Or you just replace it. It is the part with the heater/defroster controls in the dash. Not really difficult to replace.
as an example, just use the correct part !
Thursday, December 15th, 2016 AT 5:26 PM