Just a thought, but it took me ages to realise that any weight on the passenger seat made the light flash. Could this be the same in the Fiesta? The car "thought" there was someone sitting there.
Otherwise,either trace the wire to the light and disconnect it, or providing there isn't an annoying clicking noise with the flash, cover it up with a little piece of thick tape. I've done them all!
Olga Danes-Volkov, April 2008