Problem with brake/tail lights

After breaking the old one, I have just replaced one of the rear lights on my 2000 TD5 Defender. Now, with the lights off, the brake light works properly. When the lights are turned on, the tail light works properly. BUT when the lights are on and you apply the brakes, the lights goes off completely.
I am sure it is me doing something stupid, but can (gently) someone shed light (!) on the problem?

Check your earths.

check the brake light bulb, it has two filaments, when you brake the tail light filament is bypassed so if the brake light filament it bust you have no lights - guess how I know lol

had to change one last week...