discovery back brake lights.

Hi everybody,new to the forum,just had disco in for its M.O.T, a few things it failed on,my mechanic pal will be fixing them on monday. The thing is,the rear brake lights do'nt work, checked bulbs ok,fuses ok,plugged trailer in,trailer lights come on ok,so brake switch on brake pedal seems to be working ok, just thought i'd ask if anybody has had same problems,and if there is a easy solution to this problem,as i will be using disco over weekend,prior to it going into garage. any advice will be much appreciated.
thanks.

If the trailer lights work then the fault must lie in the last bit of the wiring harness or the bulb holders. Possibly a bad earth.

cheers REDHAND,will check that tommorow, bad earth sounds most likely suspect!!! thanks for your help.