Third brake light always on even if car is switched off audi A3 2005

[uk174]

Morning,
I have recently noticed that my third brake light is on even thought the car is off and parked up, however it doesnt seem to do this always. It seems to be on in more damp conditions.Any thoughts as to where to start looking with this one,Any feedback will be greatly appreciated

[Crasher]

Does it have a tow bar?

[uk174]

no tow bar, but if i disconnect the battery for 10 mins or so the third brake light goes off and stays off, but then at some point it comes on usually overnight though not always.Damp wet conditions seem to be a trigger for it .

[Crasher]

I can’t consult a wiring diagram but I am confused why one of the other brake lights don’t come on, I can’t remember how they are wired up from the BCM, I need ELSA at home!

[uk174]

once again thanks for your input, but only the third brakelight stays on. The other work when the pedal is pressed but the third stays on all the time