Without knowing what you mean exactly about "Not kicking on" it also could be a vacuum leak. The selector (Fllor/A/C/Etc) works on vacuum. A leak in a hose or the selector mechanism can cause something similar to what you experienced. My knob was pulled out a little (had something hanging on it, don't ask) and when I accelerated, the air stopped coming out of the vents, let off the gas, cold air came back. Removed hanging object, pushed in knob, worked o.k. again.
So could be any of the listed things in the other posts too.
