ok, i did a search here on the forum and i am figuring it is going to be a vacuum problem because from what i have been reading here, it seems that always is the problem with anything that has to do with the dashboards air vents not working properly.
i just want to explain my exact problem because it is kind of different than the other vent problems i have found here.

my top dashboard air vents are always blowing air at a low speed. it did not do this the 5 years i have had the truck until the other day.
BUT, whenever i turn on the blower motor it works properly on all speeds, in A/C & heater.
but whenever i have everything turned off, there is the air coming out of the top vents.
i did notice that when i put just the front defogger on, the air comes out of both the windshield defogger and the top air vents. when i direct the air to come out just on the floor, it will come out on the floor and still the top vents.
i am figuring this too is a vacuum problem, correct? i have no clue where these vacuum lines are. any help is very appreciated. its a 2004 4X4 blazer. thanks

 

Most likely is a combination of problems. It would make sense if the upper mode door actuator were not sealing the door properly that it would cause air to leak past to the vents. The lower mode door actuator though is supposed to seal off when in HEAT or OFF modes.

 

i have checked for vacuum leaks in the engine compartment. everything seems OK.
i want to check the vacuum lines in the dash for the heater controls. how do i get to these lines? it seems like on the passenger side of the dashboard underneath near the center console, there is a panel that unscrews. is there access to these vacuum lines underneath the dashboard? i'm afraid to take stuff apart if there is nothing there. Thanks

 

Try setting the vent control to the upper vents and with the fan off push the recirculate button. That seems to work on my '05 silverado

 

You can remove the hush panels as well as drop the glove box down out of the way.

 

I'll give that a shot. Thanks!

 

Mine has done this since the day I bought it new from the dealer. So has my '90 C1500. I've always thought it was supposed to, though it doesn't in some other GM vehicles I've had/have.