High-Mounted Brake Lamp Bulb -M25-, Checking
WARNING: This page is about a different car, the 2014 Volkswagen Touareg, 2013 Volkswagen Touareg, 2012 Volkswagen Touareg, and 2011 Volkswagen Touareg. However, it is still accessible from the selected car via links, so may be relevant.
Check the High-Mounted Brake Lamp Bulb -M25- with the output diagnostic test via the Comfort System Central Control Module -J393-.