For some reason the fuse for the courtesy light is very sensitive in Ford's.
I have blown a couple changing courtesy light bulbs.
Seeing as led bulbs are polarity sensitive a lot of people fall foul of this and get bemused when their lights don't work.
Btw Bunny, love the Fallout 3 reference.
When fitting or changing a bulb, disconnect the light unit from the wiring.
The twist in bulb base contacts are wide enough to short circuit the positive and negative contacts on the light unit when locating the bulb causing the fuse to blow.
Incidentally, from memory I don't think the interior lights are fused on the mk7 Fiesta - they are driven directly from the GEM which has built in short circuit protection.