Oh yeah, and there is a line for the tranmission that goes across or behind your airbox...check that. I pinched mine when I added an SLP intake system and that made mine slip a little too.

 

Anyone know what areas under the hood one might begin to look for this leaking vacuum line that would cause the control to only blow air thru the defroster vents on a 1998 Monte Carlo Z34 ????

 

any HVAC lines under the hood is where I would start.

 

I had same prob on my 2002. It was the vaccum ran down by bat on rt side. It was corroded into from bat acid.

 

This thread's pretty old, not sure if the OP ever found out what his issue was though

 

right now I'm having this exact issue. at least I know where to start looking, I've already got my dash apart, no vacuum at the "mode" switch or from what I can tell with the finger test...so going to the motor to check next, looks like I will look near the battery first. I do remember messing with the engine cover around the time this started, so possible I pulled one off messing with that.