rear lights?

i have a 53 plate fiesta "black" all in all i am very pleased but i have a problem with the rear lights, when lights are turned on the rear lights work but the brake light is also illuminated, when the lights are off the brake lights do not work, other than this all rear and front lights work,i have already tried the bulb which is mentioned in the handbook! anyone heard of this or know if i should be using a double or single filament bulb? thank you