My vent control is stuck on def. No matter what position. Any ideas?

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2001 silver Lightning
'69 Mustang GT
'82 Mustang GT (drag)
'48 Ford F1 pkup

 

vacuum tube goes from the back of the controls to the engine compartment and is hooked to a vacuum source. if not getting vacuum will default to defrost, hope this helps..
jim

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-White 2000 L, stock, currently, 17.6k miles and having more and more fun with it everyday.

-1991 ford mustang lx conv.
stage 5 Kenny Brown suspension.
302 short block, twisted wedge heads with street fighter port.
Cobra upper and lower intake.
Vortech 8psi huffer.
190lph felp pump with t-rex helper.
36lb injectors.
3.73 geared.
AOD trany customized by the local seeed shop, (Alternative Auto).
Battery Relocated.
2 1/2 inch. off road exhaust.
approx 400hp and always going up, not exact numbers not been on a dyno yet.
Hasnt been to strip, being built for the turns..