Broadstone G.C. car park /Rear brake bulb warning - annoying

[groutie]

Hello to the chap who windscreen noted me on Monday. 36 hole slog round there so didn't see your note till I was bleary eyed and done. Shame, would have liked to press the flesh. But, yes I do come here, not very often any more I'm afraid, more of a watcher now.
Whilst I am here I wonder if anyone has any ideas can help me. I got shunted a few weeks ago, nothing heavy but the result was a new rear fog light. This has been sourced and fitted, however I now get a rear brake light warning on the dash ONLY if I drive without lights! Drive with lights on, no problems..... I'm confused, the body shop who did it are confused and so are Now Vauxhall at Heathrow. Any help would be greatly appreciated