If you want to add a vapor barrier to your car door, there are a few things you can use. One option is to use a product called Mylar. This is a thin plastic film that can be applied to the door and will create a barrier against moisture. Another option is to use foil tape. This tape is made of metal and will also create a barrier against moisture. Be sure to apply the tape carefully so that it doesn't come off easily. You could also use weather stripping around the edges of the door. This will seal off any gaps where air or moisture could get in. Whatever vapor barrier you choose, be sure to install it properly so that it can do its job effectively.