The best place to start is to know why the plastics are fading. If you know, it isn't polish and/or wax residue then it is most likely losing it colour through a combination of oxidisation and the plastics losing their natural oils.

Dressing type products work temporarily to reinstall an oil to darken the plastics, but these are tempory and will eventually dry or wash out.

Solution Finish is an interesting product that I regularly use for faded trims. It's not a dye although its colour would suggest so, exactly how it works is a bit of a secret but its creator, Chris West, claims it dissolves the oxidisation effect (which causes the lighter appearance) and restore the natural oils and finish. So instead of applying a product to mask the effects of ageing plastics, this more so rolls back the years.

Now I like to have something on my plastics to protect them and stop the fading happening from the outset. For this, I use CarPro DLUX which also darkens the finish. It can be enough for moderately faded plastics but cannot rescue badly faded or patchy plastics.

So the obvious answer is to apply Solution Finish then top with DLUX (or the likes), BUT after some testing and discussions with both manufacturers, I cannot be confident that the DLUX will bond with solution finish beneath it. As some have covered above, having the plastics clean and free of any interfering oils or dressings is key to durability.

to conclude, if they are lightly faded, opt for a ceramic based product and expect to reapply every 12-16 months or try Solution Finish but expect the plastics to fade over time as the oxidisation effect takes place.

Hope this isn't info overload :)