It's long been a problem not only specific to MOPAR
but the black plastics used on exterior trim items
tend to fade in time to greyish blacks.
I have just used a product sold as "Wipe New"
detailer and the blacks look better than when
my DJ was new.
It's not the cheapest way to do it nor least time
consuming but if you follow the instruction to the
letter the results are excellent , now to see how
long it lasts , the manufacturer says about 2 yrs.