have the same problem with a 2003 Focus RS. We thought we'd try new rear light seals, but Ford don't list just the seals, you have to buy the whole light unit!!!
Did wonder about getting a sheet of the same foam material that they make the seals out of, but I don't know exactly what the stuff's called as there's many different types of foam available.
Hadn't thought to look at the two other locations so we'll give them a try