AC & heat sporadically failing, for years...

I've got a 1997 GMC Yukon (bought in 2001). Anyways, it's always had this problem: the fan starts blowing, I can feel the faintest leak of hot or cold air out of the vents, but no air comes out of the vents. This is an intermittent problem. Sometimes it works fine, sometimes it doesn't. No telling when it will fail or when it will work.

Of course, over multiple trips to the mechanic it has worked every single time for them.

This is driving me nuts! At this point, if there's a part that's a likely culprit I'll just buy it, hand it to the mechanic and have him replace the !#$@ thing. Does anybody have any ideas what could be failing that would keep that nice cool (or hot) air from getting all the way into the cabin ?

Thanks in advance,

Pat

Had a '97 Tahoe that had the A/C Heater control fail. It would work once in awhile then finally quit altogether. Found a replacement on eBay and put it in myself. Easy to do. Otherwise it could be a blower motor resistor.

OK, this thread looks close to what I've got, so I'll jump in here. I just sold my '90 Grand Wagoneer and "upgraded" to a '99 GMC YUKON. The Waggy had daggone good heat, and Ice cold air. It's cold in Carolina, now. But, so is the air coming aout of my Yukon's vents. The truck is actually my wife's, so now its getting colder in the house.

The Engine gets Hot.
The Heater Hoses to the fire wall are both hot.
The floor, cab, defrost blender doors seem to be working just fine.

Is there a separate mixer door for the heater?
Has anyone else had a similar problem?
Controls or heater door?
How hard is it to access this thing? I'm, BASE certified (Backyard ASE certified), so I expect I can figure it out...but, should I be scared?