Vauxhall Tigra brake light problems

When the lights are on, the brake lights come on as well, even if you are not pressing the brake pedal (pressing and releasing the brake pedal makes no difference). When the lights are turned off, the brake lights apper to work normally, but I can also see the rear lights come on. This is a crossed earthing problem. But I am no good at auto eletrics, please help.

Thanks, Seb.