If not frozen fins, your A/C is leaking around the compression gasket. There is a square foam gasket that is placed between the roof A/C and the roof. It seals the unit to the roof. Over time, they compress and will allow water in, both rain and A/C run off. If you remove the inside cover, you'll see four 7/16" bolts, one in each corner. You need to SNUG these down. This should be done every year or two. I like to use a 1/4" drive socket set so that I don't use too much pressure. Again, snug, not tight.