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Air flow thru defrost

i have a 94 gmc truck.air is constanly coming thru the defrost vent.i heard this is a vaccum related problem.i need pictures of where the line comes thru the fire wall or a location of the actuator under the dash,to be able to trace the line out.doea anyone have a picture to post./thanks

You need a service manual. It will contain the information you seek. Book stores sell manuals such as Haynes, but factory manuals are more inclusive. Used manuals of all types can be found at