Gary,
If it's not the Prest-o-change one that you can change rapidly, then I used black electrical tape on all 4 sides to secure it to the back window directly in front of the 3bl itself. On the rear window itself is a clear spot that lets the 3bl light show thru, but all around it is the dark tint so you can't really see the tape that holds the cover. Hope I explained that so you know what I mean.
