Is it the round/hexagonal tibbe-type key or the more modern flat type?
My mk1 focus has the tibbe keys and the lock is very worn on the trunk and doesn't turn easily without the key spinning all the way around (despite having a new key cut) and WD40 makes it worse in my case it just spins easier.
I believe to get to it you need to take the inside door panel off, and assuming you've got a mk1, there is 2 or 3 bolts holding on the trim strip above the license plate, those need to come out and everything else should then be accessible. I can look up diagrams later on if you like from the Ford software
I tried to unfreeze mine and it wasn't happening, so I had Ford make me another one for over $100. Alternatively, you could remove the cylinder and take it to a locksmith, leaving a copy of your key as well. They can get the barrel out and probably replace the tumblers too. When I was a prankster in high school, I took my school-issued combo lock to a locksmith and had him make a copy of the master key that would open every locker in the building. They can do amazing things, and even a teenager can afford it.