I have bought 94-94's for parts and the trunk floors are rotted out on them and you can see inside the frame rails. This is only with about 100-130K on them.
I even looked at an 03 Cobra with rust under the door...and only 34K on the clock.
what did you end up doing with the car? its pretty common for the fox cars to rust there.... they rust from the inside out because ford was smart enough to leave the frame rail exposed and tons of junk gets trapped in the rail only to eat away at it. I am actual fixing on now, its not too hard if your a decent welder, and no you do not have to replace the rail, but the K- member has to be removed.