Finding and curing water leaks in your Fiesta
If you own a Fiesta for a long period of time then sooner or later you may find yourself with a soggy interior which, if left unfixed, can result in your car rusting from the inside out. Identifying the source of a leak can be a pain but the information below should give you a starting point of where to look. If you cant identify the leak then removing interior trim, drawing a chalk line around the rough location and directing a hosepipe at it can help to show the source, the chalk will be washed away where the water enters.
Front footwells
Split heater matrix. A leaking heater matrix should be easy to identify as the source because your coolant level will keep dropping quickly. These can be replaced with the dash in situ but for the thirty minutes it takes to remove it, it's probably easier to take it out.
Rot at the bottom of the A-Pillars. Its common for the 'pockets' of metal behind the dash at the base of the A-pillars to rust through making a hole between the cabin and the area inside the scuttle, its not an easy area to repair but taking the dash out should show it up as the leak culprit.
Sunroof drainage pipes. The standard Ford sunroof recess has a hole in each corner, if you remove the roof lining you will see that this hole is actually the opening of a small length of steel pipe which makes up part of the sunroof recess panel. A plastic hose is then attached to these pipes and directs the water down the A and B pillars, directly into the sills (who knows who thought that was a good idea!), there are then holes in the bottom of the sills to let the water back out onto the road. if these holes get blocked (or welded over) then the water fills up inside the sill and can drain out into the interior, if the front of the car happens to be the lowest point then it will run out into the footwell, until the sill rusts out and dumps the water onto the road with a load of rusty metal.
Wet seats
Sunroof seal. If the sunroof drainage pipes are blocked the water can back up in the sunroof recess and drip into the car leaving a damp patch which will quickly turn into a mould (and damp) patch. Unblocking the drainage pipes and cleaning out any muck in the recess should cure most leaks, if not then it may be time for a new sunroof seal.
Boot floor
Water under the boot floor boards can be caused by quite a few things, the easiest thing to eliminate is the rear washer bottle, it's been known for people to check all the other causes and forget to check its not the issue. Check by placing it in a empty ice cream tub and see if any water materialises.
The next thing to check is the seals, there are quite a few culprits, the boot lid rubber seal, the rubber rear screen seal, rear window seals, rear fog light seals (reverse light if it's a Mk1 vehicle), missing rubber washes betwene the bumper mounting bolts and the shell (fitted on the outside) and the gasket around the rear light clusters can all let water in. If they dont appear to be the cause then have a look for any rot under the arches and boot floor corners, if there's a hole in these areas then water can easily splash up and enter the cabin.