HVAC problems

The selector knob on the heater/ac does not work properly (the knob that selects defrost/floor or dash vents). It sometimes works but it takes a while and doesn't fully select the setting I want. Sometimes it blows out of all the vents regardless of the selection. I know its vacuum operated but I don't see where the vacuum line comes from or goes to to fix it.

Also I have a few friends who smoke and the heater smells every time I turn it on, how do I open the system? I see screws but they don't appear to let you open the ducts to clean to the heater and AC cores.

(95 Tahoe 5.7 TBI/4x4/4 door/front HVAC only)

I am not sure about the 95, but I have a 97 that was experiencing the same problems. I replaced the HVAC unit (a little over $100 on EBAY) and it solved the problem. I didnt' have to do anything with vacuum lines on my 97, but again, the 95 may be different.