Heater works defroster does not

This can be many things, but most common is debris in the heater fan motor area. Hard to fix without removing fan motor and blowing out heater ducts, but sometimes compressed air blown through ducts can clear it out.

I had a similar problem with two different cars. Mice had somehow got into the heater ducting and built nests. This will plug up the ducting, or if nest is next to the heater fan, it can keep fan blades or the squirrel cage fan from turning. If that is the problem, you'll need to take some of the ducting apart and clear the obstruction. Good luck.

Usually a sign of a vac leak between the engine and the control on the dash or the blend door.
The defrost position is the default when there is a problem with the system so that the windshield stays clear in wintertime.

What is the engine size?

Check to see if the duct hose to the defroster vents has come loose. If air is blowing out under the dash when you have the control set to defrost, then this is likely.

If it smells like antifreeze, and leaves a film on windshield, it's probably a leaking heater core, but if not, your drain tube/passage for heater/AC duct-work, is plugged and not allowing moisture to drain out. Not 100% sure where drains are on that but, I would look real close at lower firewall in engine compartment,opposite duct-work, as well as under truck near front floor, for tubes sticking out, and carefully and gently, run a pipe cleaner or similar in & out to clean, until moisture starts to run out.

something might be stuck in the ducting not allowing the door to move and direct air flow to the other ducts. First pull straight on the nob and pull it off of the dash. then take a pair of pliers and make sure the smell metal peg will rotate.

Car manufacturers use the defrost mode as a default for safety reasons. Being they want you to be able to defrost the windshield no matter what. So when something goes wrong the system automatically goes to the defrost mode. You most likely don't have the vacuum hose connected under the dash at the climate control assy. Or if this vehicle doesn't use the vacuum hose style you may have not plugged in one of the connectors under the dash or behind the climate control assy. Good luck.

Some times you can "reset" the computer for that by disconnecting the battery for 15 or 20 seconds then re-connecting it. If that doesn't work then the mode door motor assy might have stripped gears or broken gears. (they are plastic) If you are familiar with where the mode door motor assy is you can turn the knob to change the mode (i.e. defrost, panel, floor) and see if the motor is turning. If motor is turning and mode does not change then the plastic pieces have broke or stripped. Usually when there is a problem with the climate control system it will default to the defrost simply as a safety thing. So you will have defrost so you can see where you are going. Hope this helps you all out, good luck.

With the engine off, radio too, turn the key on and try the heater fan on all speeds. No fan, check the fuses. Fan is ok, check the coolant level, if thats fine, then check for a mouse nest in the duct work. Frankster016