I have been wrestling with my black bumper, and side trim and mirrors, etc for the last few years that I have owned my 98... but I think I have found the Holy Grail and it was filled with something from Meguiar of course.
I have been using the Hot Shine Tire Gel on those pieces and no amount of Oregon rain and rain and rain and rain turns them grey again.
I dig this stuff. I even made my 8-yr old daughter come out and video the instant results as I applied just one coat a month ago. I have been waiting to see of the pieces started to melt or something before I endorsed it, but so far it looks showroom still.
Just an FYI.