ford fiesta mk6 please help

[greenjacket1365]

i have a mk6 fiesta and having problems with my rear lights.when i have all my lights off and press brake pedal ,brake lights come on but lights on dash do as well and also my front side lights.when i put my main lights on brake lights stay on all the time as well as normal lights,i have disconected each rear light in turn and found that when i disconected ns light and pressed brake pedal lights on dash and side lights did not come on so assuming problem with ns light.have checked for bad earth on this lights bulbs ect but couldnt find anything.my wife has a fiesta but 5 door and lights are bigger than mine on my 3 door.i took her light off and all though i couldnt fit it properly to my car i plugged it in but when i did this the osr light wouldnt work anymore but every else worked fine.just wondered why this was and if i bought a new rear light for my car would it sort the problem.thanks