If it's that rusty on the cab side how rusty is it on the other side where the moisture is actually traveling? I'd try removing/cleaning up all the rust around the vent panel so I know how pitted it is and then I'd be pulling the vent door and looking with a mirror at the hidden side and seeing what that looks like. It looks to me like moisture is sitting on the lip that's formed on the back side of your arrow and that would be where I'd try to clean/seal things up. I'd stay away from silicone as it creates problems if you ever want to paint.

Give me ambiguity
or give me something else