I have that typical air goes to defrost problem it seems like everybody gets. Problem is, I have checked the lines behind the battery at the firewall and they aren't damaged. Remember that I have a supercharged application and I wondering if that is sucking out the vacuum? Any ideas?

 

I had the same problem on my 2002 screw. I checked and traced the lines from in between the battery and firewall all the way into the dash and to the climate control. I have the push button electronic type, but it is still supplied and runs off of vacuum. I had vacuum pressure at the control and after checking, it turned out to be a bad switch (AC button). All other buttons worked perfect (vent / floor-pnl/ defrost / floor ect.. I went to a local wrecking yard, got lucky and found the identical control, plugged it in and everything is fine. 3 hours of work (could have been 30 minutes) and $40.00 later, I'm driving in 40 degree air with it being 100 outside. Hope this helps.