its not a vaccum operated heater door, its operated by an electric motor. to know if it is the motor drop the glove box door all the way down and look for a little motor enclosure sitting on the heater box - about 2" x 2" if i remember correctly. pull the electrical plug and remove the two screws holding this gear box on. they are really really hard to get at but you can do it. once the assy is off move the door flap by hand and start it and see if you get heat. you wont be able to move it after the fan is on high due to the vacuum it creates on the low side. also try the door in the other position too.
there are other threads on here about this. i found on mine the gear was cracked - happened when i went to turn the heat on after the coldest morning of the year last year. open the housing and see if yours is cracked too. if you know someone who has access to a lathe you can have an aluminum insert machined, press it in the gear face recess and use it to the hold the cracked gear together. sounds strange but trust me it works. or buy a motor for about 60.
thats what i found in my 98 and others here have had the same pro as well. this is assuming all coolant and engine related things check out.
good luck