My gaskets look decent, not brittle and falling apart. Any suggestions on how to best clean them up and restore the dark black look before I re-install the taillights? Thanks

I've used carberator cleaner(B12) on rubber pieces to clean, and soften, them up. Be careful, though. Just dampen a shop towel and wipe them down. Don't use too much. Let me know if it works for you. I've also tried laquer thinner but, this will soften the surface of the rubber too much.

Steel wool and a lot of elbow grease works well too. I did mine 20 years ago and they are still jet black.