One of the few minor annoyance of the Promaster including high swivel seats as I documented before is there is no rain gutter on top of the doorway, and as such if the sliding door was even cracked a hair, rain would start pouring in the door and into the van.
People have found extremely quick, ridiculously cheap, and more than sufficient.
1. Find a plastic corner protector at your local hardware store. I brought this from Amazon - Trim-Lok Drip Rail
2. Cut to a length that is longer than the opening in your sliding door.
3. Secure above your doorway at a slight angle.
4. Enjoy opening your door without inviting in a deluge of water.
Here's a good blog and quick video that shows the rain gutter in action!