I happen to have my instrument cluster out right now working on an electrical issue and I can see the vent hose runs directly behind the instrument cluster and attaches right behind the key switch. If you push your cluster out from the back, you might be able to get a look and see if it has come off. (Drop your bottom panel and look from the bottom first). Also, check under dash to see if you feel air blowing down there from a disconnected hose.

Best of luck. KRW

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1991 350SDL 350k+ miles