Have a seal and light I'm looking to replace.
Got some aftermarket chinese junk, that just does not fit well.
Would be great to find an on-line parts catalog, like the factory has.
Dealer is no help at all.

Looking for......
1) The foam rubber seal for the third break light/cargo light.
It goes under the light assembly.

2) Under the tailgate, on the bed body, there is a light bar with three red running lights.
Can't change the bulbs, need to replace the whole thing.
One is burnt out, but want to change all three.
Found some that kind of look the same, but the plug is different.
2000 K3500 4x4 LS extended cab, dually.

 

You can use some foam weatherstripping sold at hardware stores and cut it to fit for the third brake light. For the light bar you can use the one that kind of looks the same and cut off the pigtail from the old one and splice it to the new one. This will be less stressful than trying to find the correct parts. Also less expensive.