I tried to do some searching for this issue. I will do some more later but I am pressed for time today.

I have a 91 Burb 2wd(158k) 350TBI. I just got this about a week ago. The defrost was actually working. The next day the blower motor stopped working. after tapping it it starting working again. Now the issue is the heat only blows out of the vents no matter what setting it is on. I am sure the problem is the vacuum line or lever(don't know proper name) that controls the position of the air flow is off.

My question is where is it located so I can check it?

Thanks in advance for you help. Like I said I will do some more searching on this myself as soon as I can get settled and can search like I want to.