99 Durango blend not working????

Just bought a 99 and found that the blend will not switch from the defrost position.I know this is a vac switch but I dont know what to do to fix the problem.Is it a switch problem or could I be not getting vacum to thew switch?I dont even see where the vac comes from to supply the switch.I hope some of you have been down this road before and can point me in the right direction.