Assuming (which has come back to bite me on a few recent threads :nervous: ) you're talking about rubber trim as opposed to black metal....
I really oughta deal with this properly on my Tahoe, but all I've done so far is clean it up and maintain it with ValuGard's Fast Finish after every wash.
I cleaned mine up with Hi-Temp Heavy Cut/"357" followed by KAIO. The 357 is one of those rare compounds that works OK on most rubber/plastic trim as long as you don't let it dry, and it got rid of the really nasty oxidized-white outer layer of rubber. The KAIO did its usual cleaning and, well..."good enough" for the beater SUV. I would've normally KSG/845'ed it, but I didn't bother; I redid the KAIO intermittently for a while but now I'm merely doing the Fast Finish (a very good QD for stuff like this).
If you really want to "restore" it properly, you could get into the ValuGard ETR kit, but maybe just a good cleaning/maintaining would suffice.