Ford Ka

Its pry something real small and stupid. The only thing I think that could be the real culpret is a bad ground. Fords are really hard on the electrical grounds. I have no clue how old your car is but if it's like 10 years or older then you pry have a bad ground. There is no good way to test for this kind of thing. If you could get a wiring diagram of your car you'd pry have better luck finding where the ground is for that circut. A ohm/volt meter would be handy also.