Defrost is the default position(no vacuum). The reservior is located next to the battery, you would have to remove the battery to replace it if bad. Here is a pic of it.
Make sure you have vacuum supply to it from the intake manifold and then make sure you have no leak there. It supplies the controls on the black hose.
Here is the vacuum schematics
If the reservior is good and you have a good vacuum suply to it and also to the control head then your problem will be the control head itself.
Maybe this will help
Vacuum Applications Chart-Electronic Automatic Temperature Control
V = vacuum
NV = no vacuum