Help!! - Nissan Micra 'T' : Brake lights permanently on?

Hello,
I have a 1999-T Micra & the brake lights are permanently 'on' & the corresponding fuse is permanently blown.

I replaced the fuse a couple of times and as soon as the ignition is 'on' the lights come 'on' and the new fuse blows.

I have checked the switch on the brake pedal and all the fuses under the bonnet - All appear to be OK.

Any advice would be much appreciated.

Thanks

Chris