Ok I'm having a problem with my 98 Blazer. This summer I did the mod to keep the heat from coming out at the floor and frying my feet. The other day I unplugged the vacuum line and plugged it back into the place it goes. Now the heat or a/c won't go to the floor or defrost vent. It will swap between hot and cold almost instantly though. What could be causing this?

 

No idea what "mod" it was that you did... Are you sure that everything is plugged in properly? Do you have auto temperature control?

 

No I have analog controls.
The mod is unplugging the grey hose behind the glove box and plugging the clear elbow. That's the only thing to the mod and yes it's plugged in correctly. I'm going to replace my vacuum hose under the hood tomorrow.