2000 F250 Heater Problems

I really need some help on this one. I have bought an F250 Powerstroke and the truck is in great shape, but the heater switch will not funtion to switch between defrost and floor and so forth. It is vacuum controled and all lines seem fine all the way through the truck, but there is NO vacuum going to the control switch itself and I don't know the first place to look, can anyone lend some advise? Thanks.