The rear brakelight switch has a sprung blade as part of it (from memory it was 10yrs ago I had to sort mine). I got a bit of gravel lodged in mine which caused it to stay on IIRC...
Could be the tail lamp shorting across brake and tail filaments - you wouldn't notice if the tail filament was on when you applied brake - but you'll certainly notice when you put on tail lamp and brake filament is also on.
Had this fault with an H7 headlamp bulb years ago - both filaments shorting together in base of lamp causing both filaments to be on together.