You have a 2000 and you have the slide adjustment? I was under the impression that 99-00 had the turn knob for temp control?

If you have the knob, I'm not sure why you wouldn't be able to turn it to cold.

If you have the slide (pre '99) which is operated by a cable (at least it was in my 98) then check behind the console/bezel to see if anything is messing with the cable. If nothing is messing with the cable, disconnect the cable from the control and see if the slide works period. If it doesn't slide, there you go. If it does, then your problem is somewhere else probably.

Just some ideas