My 300D saw it's first rain today. I opened the trunk and found some water. The best I can figure it must be the rear window seal, it didn't seem to be coming from the trunk seal. While I was looking around for the source of the leak I pulled the plastic shroud/covering off the side by the radio antenna and noticed this hanging there. It looks like it would carry air as the end is threaded, and you can screw it on something. It has a little push valve in the center of it, at least that's what it looks like. Kind of like a bicycle tire pump attachment that screws on.
What is this thing
I have attached a couple pics. Thanks for your help.