ive recently been fixing a leak on my friends mums focus mk1 the dreaded rear light leak ford put a new seal kit on it which didnt help in fact it somehow got worse so i took a look removing alll the trim i didnt take pics as the weather was against me and i had to be quick buty i found a few points one of which on the mk1 you dont need to remove all the trim unlike the newer cars the back of where the light sits is double skinned box section so you cant access the rear of the holding clips like you can on the newer focuses.
1.with the rear light removed the upper portion that fits against the roof edge has had a seal fitted from the factory like a pollen filter seal, that had degraded and was almost non existent this prevents the rain from creeping down the edges and right under the light so i removed the old one and replaced it with a new one a pollen filter seal trimmed to fit does the job very nicely
2. the plastic fixings dont prevent the water from dripping in around the edges of them when its heavy rain so exterior frame sealant was used to cover the edges of these fixings its overpaintable with a touch up pen so no white bits showing through you can buy new ones from ford and apply the sealant to the area first then pop the fixing in place so its well sealed i did all the fixings the main screw one and the one at the roof corner.
3. the small fixing screw was wrapped with 4 layers of ptfe tape as was the main screw for the interior screw fixing on the big thread for the interior screw ( only use ptfe tape on the top area of the thread).
all was put back together and tested point to note dont overtighten anything i noticed that ford had made the inside screw really tight as a result it was pulling the fixing screw out its clip leaving a gap for water to get through its only holding the light in place so dont go too much with it give the light a shake if it isnt moving its tight enough. now theres no leaks its put up with torrential rain as well as a major soaking with a hose pipe and has been fine.
the fault is simply the design a securing screw with no rubber seals etc will let water in eventually and with the upper seal degraded the water runs under the light rather than down the rain gulley like its supposed to same as the washer jets with no rubber seal they lkeak forcing ford to redesign them with rubber seals im surprised the rear light seal kit hasnt been revised but there you go.