Hi, you dont say what year Fiesta, not sure if later ones have three brass contacts between door and post like Transits have, or could be a break in the wiring between door and post if its older, a resistance check might help. Or if you're anywhere near east Kent get back on here for a good auto electrician i can recommend.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.