I have exactly the same problem! The only solution that makes any sense to me is replacement of the entire assembly. This in fact is next on my to-do list and is going to occur this spring once it warms up. If you are interested I will let you know how it goes. The door will get repainted, new weather stripping, and new door panels at the same time, and hopefully the loose window crank and door handle will get secured properly.
I bought a replacement door for $100 Canadian dollars just for the vent assembly. But a full replacement assembly should be available from suppliers. As far as I can tell, several screws get removed and then the assembly lifts up and out of the door with a rotation.
If there is a better and cheaper solution I am interested.
