There is a wavy metal retaining ring on the back of the vent that holds it into the dash. Your's must have fallen off or broken. And the vent hose just slips onto the back of the vent. You'll have to take off the under dash panel to get in there to put thing back together. If just the front part came off vent, it clips back on or goes on and turns slightly to lock it into place. You'll have to remove it to get it back together. Which would involve removing the metal retaining ring on the back side of the dash.

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