Hello Tilly,
Tilly there's a switch under driver side, that is controlled when you apply the brakes, look close to where the brake pedal shaft is located, there will be probably a plastic unit with a button on it-- as the foot brake is pressed down it works the brake lights.
This switch can fall off it's mount or not be aligned properly so as to keep it from working. This is where I would look first, if this is ok--look at the wiring in the circuit.
Also, let me add the switch can be in it's proper place but is defective (not Usually though).
Hope this helps
In Christ,
walt, October 2008