Well the tail lights from 廣東路; 广东路 that came with my truck started filling with water.
No great shock. I could see condensation along the sides where the glue was letting go when I bought it.
I bought a set of clean GMC tails off a 2001 Sierra in a local salvage yard along with the lamp pigtails that run down to the junction block near the LH frame rail. Four plugs... 2 Tails, 1 Trailer, and 1 Vehicle Harness. Much easier than the T400 and earlier trucks.
Do them one at a time. It looks like you can plug the LH tails into the RH J-block socket. It would be annoying to have the RH turns activate with the LH selected.
You can polish lightly oxidized plastic lamps with rubbing compound on a foam Roloc pad mounted on an air die grinder.
I installed Clear Lamin-x to protect them from further UV damage. You can't see it til you get up close.
The Amber turn tails used on the 99-02 Silverado & 99-07 Sierra don't have the peanut bulbs at the bottom that the 03-07 Silverado has.
If you buy new tail lamps you can get them with the lamp pigtails included. This is a pair of 03-07 Silverado lamps with pigtails. The Sierra lamps are available the same way.
Not sure how long the lamps made in 廣東路; 广东路 will last. Some work well and last and some don't.
Mine are used so of unknown pedigree. They have TYC-5186 & TYC-5185 molded into the back plastic and DOT compliance mark on the visible surface. They don't have a GM part number on them. They don't leak and they are old enough that they were oxidized somewhat.