Since my car is exposed to the elephants... e-r-r-r, elements, a lot of debris collects on the lip around the trunk, and then goes down to where the taillights fit in. Since it was a nice day, I got a little obsessive/compulsive and decided to pop out the taillights to see what lurked underneath. Yes, once they were out, there was a fair amount of grunge on both the assembly and the frame, easily removed though with a sponge or towel and warm soapy water (wax if you're really possessed). Everything looked okay with the left one, but with the right I found a patch of (mostly, I hope) surface rust running from the corner of the where the light goes, over to the rubber seal by the trunk. I think it can be easily dealt with, sand, rust converter, prime, paint, but I also think I'll have to lift a section of the rubber seal and peek under that to see how far it has extended.
Anyway, my point is, it's a simple enough check to see if a nefarious little leak has been working toward ill ends in a dark corner.
Removal of the lights is straightforward, undo four nuts, disconnect electrical connection (press tabs and gently use a screwdriver), and I suggest using a rubber hammer on the four exposed studs, the lights do form a rather tight seal against their rubber trim, and you otherwise really can't get enough pressure with your fingers to push them out, and certainly prying with a screwdriver is a no-no.