Dreaded heater control vacuum switch problem
 

Yep. I got issues with the vacuum switch in the dash of my 95 Jimmy. I went to a pick a part place and pulled out a couple spare rubber switch...whatever you wana call it. I found that the two I pulled out seemed to have more clean looking ridges than the one I took out of the dash. Specifically it is the piece on the back of the switch. It seemed to work at first. But again today I can hear air leaking through that switch when I turn it to vent or heat. I'm telling you it's frustrating. Gotta take it out again and look at it and see why it's hissing again. Because it won't open the vent or the floor heat again now.