Your heater/ac vents are controlled by vaccum. If the system loses vaccum for any reason or the selector switch fails, defrost is the "default" position.
At the selector switch you will want to see if you have incoming vac. To the switch, engine running. I believe it will be the black hose but check them all for vac. If no vac to any of the hoses, you will have to trace the hose back to it's source on the engine intake manifold or a vac junction block on the firewall looking for breaks or disconnects in the line.
If you "DO" have vac to the selector switch, rotate the switch checking for vac at each output port as you turn the switch. If nothing, replace the selector switch.
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Thursday, October 9th, 2008 AT 12:12 PM