How do I find out which door actuater needs replacing. on 1998 Cadillac DeVille

The heat does not come out of the dash vents at all. Works good in the floor and window vents. Could it be a vaccum problem. Would like diagram of actuater door location, and hopefully how to narrow down the location. Thanks for your time. FRANK

Asked by for the 1998 Cadillac DeVille
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