What is the adhesive used on the metal tabs of the center brake light?

 

not sure of what you mean; but if you are trying to reattach it to the window, people generally use rear view mirror glue to secure it to the rear window

 

The original glue on mine, which came loose this summer, didn't look like rear view mirror glue does when a mount drops off the windshield.

I cleaned both metal tabs of flaky glue, gently cleaned up the spots on the window, and used general purpose 5-minute 2-part epoxy to put them back on. If you don't like having to hold the things for a couple of minutes until the glue sets there is a 1-minute version.