A hammer. If there are no windows, there is nothing for residue to build up on. 
In all seriousness, I have never had that problem. That may be because I leave a crack for the vapor to escape. As stated earlier, I need to see while I'm driving.
In all seriousness, I have never had that problem. That may be because I leave a crack for the vapor to escape. As stated earlier, I need to see while I'm driving.