Both my tail lights areas are leaking. Getting rear ended in the past may be the cause. The water is coming in from the little crevices and spaces in the steel, hard to say where the original source is. What sort of substance would be a good sealant?
I have a leaking tail light in my Firebird as well. It looks like a pain in the butt to get the assembly apart without cracking the fragile plastic, though. I might just have to get an entirely new tail lamp assembly. I did have good luck with my Acura, though, just taking it apart and re-sealing it.
My 1998 CRV had rainwater weeping into the tail light assemblies. Fault was the old rubber gaskets at the bottom bulb sockets and the sockets themselves. After a few times of getting the water out by absorbing it onto facial tissues maneuvered through the bottom bulb hole, I considered using a little RTV sealant but ruled it out as I thought I'd have difficulty replacing bulbs later on. Plus that I'd have to renew the RTV each time I changed the bulbs. So I merely drilled two 1.5 mm holes at the bottom of each tail light housing to allow the water to drain towards the outside. Problem solved.