The two screws are inside the trunk not outside on the light. It's an interior job...Check this person out - pictures and all:) http://cindyli.com/site/comments/vw_beetle_third_brake_light_replacement/
You have to open the hatch and completely take the inside cover off. you will find a screw on each side of the light . remove the screws and unplug the electrical connection and replace with new one. Aprox cost from dealer about 75.00 for replacement light . Pretty easy fix and I'm no Mechanic
Have you solved this problem, if not please ask your question again or you might find a free repair manual here: http://bit.ly/autozone_repair_info or go to http://bit.ly/alldata_repair_manuals or http://bit.ly/mitchell_repair_manuals_online and get an online repair manual subscription.