Why does my trailblazer stall when using the defroster?

Classic symptoms of your battery being disconnected for what ever reason. The clicking is your vent actuators that are now bad (happens on occasion when the battery is disconnected). The engines stalls when you turn your DEFROSTER on right, not necessarily the heater. You'll find the stalling when you turn your defroster on OR your A/C. To fix this problem, disconnect the battery again, and clean your throttle body, this will cost you only a can of Seafoam spray about $8 to fix. Hook the battery back up after clean, drive around with HVAC all on OFF!! The computer will relearn the newly cleaned TB and your stalling problem will go away. Fouled spark plugs are a seperate problem. Make sure you're running AC Delco 41-103 plugs. The I6 engine loves these.

Computer will reset after driving it around for a 15 min or so. Make sure your throttle body is clean, otherwise won't run correctly and possibly stall if your AC/Defroster is on. Cleaning it will solve this.

Check alternator and battery.

You probably mean your defroster, as this is a very common problem after the battery is disconnected. Your defroster uses the A/C to dry the air. Clean your throttle body with battery disconnected. Battery must be disconnected for at least an hour, but if you take the throttle body off (best way to clean it) 1 hour won't be a problem. Computer learned some bad habits, when you hooked the battery back up, it though it was getting good signals from a clean throttle body. Your throttle body is filthy, therefore you have the exact problems you described. When finished drive around for 15 min with all HVAC, A/C controls OFF. This will allow the computer to relearn a properly functioning tb.

Any codes? Fuel filter and air, dirty throttle body.

It will tend to stall when you have the A/C on and or the defrost, not the heater correct? The defrost uses the A/C to dry the air. The battery was probably disconnected or changed recently. Clean your throttle body with throttle body cleaner while the battery is disconnected. When finished, drive around for 5 miles or so with the A/C and Defroster off. Your computer will relearn the the newly cleaned throttle body. This will fix this very common problem. Will cost you about 7 bucks for the cleaner and you're gas mileage will be better.

First the throttle body needs to be cleaned, grime in this area prevents the throttle plate from closing completely and allowing air to enter, making the air/fuel mixture lean.
Engine running too cool means that either the thermostat is stuck open or the coolant temperature sensor is defective. Both should be replaced since they are in same area.

if this happens while the defrost of ac is turned on, it could be a seized ac compressor which will cause the serpentine belt to squeal and drag making the engine sometimes stall. if it happens with the ac off and the selector on say the floor, or dash vents, the issue could be a bad alternator and battery. this is confirmed when the blower motor engages and the lights dim and then the truck goes dead.

Try cleaning out the throttle body with carb cleaner.

I don't wanna laugh but the Vibration of the defrost is probaly wiggling a connection --most likely a harness ground on the engine bay firewall ---I'd check the grounds then the throttle position sensor connector connection and then the --Don't laugh distributor connections to inlude the engine to cab ground cable ---braided bare cable from the block to the firewall -

--the COIL pack may have a poor connection ---and I believe if correct on blazers the feed wire to the coil from the harness
is thick and pinkish that connection if loose describes the symptoms exactly

nothing works without a sufficient GOOD gound --loosen and retighten and ground wires-positive wires and connections ----GM is well known for wiring harness problems ---my 84 blazer
happened to share the same ground for the Blower motor and the distributor ---not anymore!
check your connections and those grounds --am I repeating myself ---and also continue to seek help in here if still persisting after that ---should only take 15 minutes to check it all
30 to fix most likely
hope this helps
HAWK