DV52, are you there? Paging DV52...
(He's the wizard of all things to do with the "Central Convenience" module and fiddling with the lights in particular).
Edit:
Dug out his über-thorough
Excel sheet of ALL the lighting functions (a mere 2385 adaptation channels) in the car that he compiled. He put it together to compare the function of each light (and thus setting) across the VW's various markets (e.g., NAR, RoW, Australia (he's Australian), etc.).
All of which is pre-amble to: Within VCDS - Central Convenience (9) - Adaptations (you probably have to enter the "security access" code first...I think it's 20103 - either that or 31347), and then do a search for
Leuchte22BR MA57 - that's the third brake light...i'd think that if that mod was possible, this is where it'd be configurable.
(And puh-leeeze...make a backup of your adaptation channels and log your changes, just in case...)