More than likely it's the cab clearance lights. This problem has been coming up occassionally since the beginning of the 3rd gens in 2002. It seemed like there were lots of cab clearance light leaks in the early days; not so much now.
Anyway, hopefully it will be a simple fix. Examine each of the clearance lights to see which has moisture on the inside. That's probably the one that's leaking. Remove it (2 screws) and get yourself a new one (they're inexpensive). When installing the new one, apply some silicone sealant to the threads of the screws prior to install. Also be sure to carefully clean the sheet metal on which the light sits with a little isopropyl to give the sealing ring around the new light something nice to seal to. When you screw the new light on, be very careful not to strip the little plastic inserts in the body.
If you want to be even more diligent, replace the little plastic inserts in the leaking light. On installation, apply a little silicone sealant to the back side of the insert before installation and let it dry for a little while before screwing on the light.
Not a difficult repair, but kind of annoying. It would be nice if there were a more positive sealing mechanism for these lights. I'd have opted not to get them if I had a choice back in 2003.
Ryan