Hissing noise behind heater control

My 96 jimmy just started making a hissing noise behind the heater control switch. I can't select any of the vent positions anymore and it only stays on the defrost. I took the heater control apart and found some vaccum lines behind it and disconnected that from the heater control and the nosie stops but put it back on and it starts again. With it apart i put my thumb over each hole on the lines and one of them is always sucking in air. Anyone know what is wrong

sounds like the switch went bad. everything you describe is correct. the dash controls are operated by vacuum. one of the lines is incoming vacuum and the switch controls which line the vacuum goes down. the defrost position is the default position. it will go there with no vacuum. if you are getting vac to the switch, but have no dash functions, id replace the switch.