Need some help! When I closed my driver side door and this piece fall off. Anyone know what this is for?

Do you know how to put this back on? It looks like it is just glue on?

Thanks for your help

 

That attaches to the car, not the door. Plus you are missing the plastic door trim. See the passenger side for reference

 

My second picture on the post is the back of the plastic door trim. The piece attached to the car is still there like your picture.

Thanks!

 

That's called a "support". Looks like PN 997-537-331-01-01C. It mounts to the door as you suspect. Not the car.

I'm not sure how it mounts. Online parts diagrams don't show any separate mounting parts. Perhaps it's glued on, but the way the holes line up makes me think there's a mounting piece that might have broken and fallen down into the door. Got any new rattles in the door?

They're cheap if you need to buy a new one. About $30.