I've been doing some customizing of the third brake light -- a story will be submitted to Rolling one of these days.

Anyway, there are two ways to open it. First, you can use either a really thin finger, or a screwdriver bent at a right angle. Imagine a tab, exactly centered, that extends under the housing (at the bottom margin) and running forward (towards you as you face the rear window). You have to slip the tool under, and then push the tab upward. That releases a catch, and the outer housing can slip up and forward (toward you).

The other way, which I personally find easier (I've done this on both of my '93's), is to simply slide the whole unit upward parallel to the glass, and off the bezel that is glued to the glass. Then you can turn it around (to face you) and see how to release the tab and remove the back housing to get at the bulb's reflector, to access the bulb.

Good luck.