Blend doors won't move - stuck on "vent"

[Alex V.]

The climate control on my '85 C3500 (454/TH400 w/A/C) has been inoperable for the last 5 years - ever since I got the truck. The fan and temperature control works fine, but it only blows on vent - won't route the air up to the defroster, etc. I've never been into the dash on one of these trucks, so I don't know where to start. I'm under the impression that the valving/actuating mechanism for the blend doors is run off of vacuum - am I right? Is there a main vacuum supply line from the engine I should check before I gut the dash to get to the switch? Also, I'm assuming the switch isn't a breeze to get to, the way the dash is designed.