92 Defroster problem

My 92 Chevy Z71 Defroster is not coming on.. The floor, bi level, heat works fine but when I press the defroster button warm/any air does not come out of top dash.
One time after about 10 min it come on..
Can somebody spoon feed me on what to check. Give me exact location were I need to check.
snipped-for-privacy@usit.net Tks Dave Tenn
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sounds like the servo motor that moves the door is not always workin get a shop manual and it will give you the location but its under the dash and be assured it'll be a bitch to get to.
Dave.

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