Same problem with my DS. I bought mine Certified Pre-Owned and it was one of the loaner vehicles at the dealership. I think they must have been using a brush or buffer on those plastic pieces. I hated the way it looked, so I started searching for ways to fix it, and this is what worked for me:
First, I bought a small bottle of Menzerna SF3500 polish and a few black foam finishing pads (Griot's Garage). I had an old 3M headlight restoration kit that included a disc pad holder that you use with a drill that I attached the finishing pads to. I also had a small amount of 3M rubbing compound (03900) leftover that I eventually ended up using as well.
Tape around the pieces with painters tape to cover the rubber gaskets - you don't want to contact them if you can help it. Contacting the tape is also not ideal but it is unavoidable. It is the lesser of 2 evils though.
Clean the pieces to get any dust/debris off.
Put 3-4 pea-sized drops of the polish on your pad and blot it around on the trim, then start polishing. I just used my cordless drill. I do not have a dual-action polisher, but it is better if you do according to the entire internet. I applied pretty good pressure, collapsing the pad to less than half its thickness or less, and did anywhere from 5-10 passes. The polishing compound is pretty forgiving fortunately - you can't really do much damage with it, so no need to be shy.
I then went over it with a dry pad that had no compound on it for 2-3 passes. Wipe with a microfiber cloth and see how it looks. It may be all you need.
For me, it was not. Using just the polish alone did get out the majority of the scratches, but there were some deeper ones that it couldn't quite handle. I used the rubbing compound that I had with the orange compounding pad (that was included with the headlight kit). Did 3-5 passes, and wiped it clean. Then did a few passes with the Menzerna polish followed by the dry pad again. While the rubbing compound will get rid of those deeper scratches, it also leaves a very faint haze on the trim that you might not notice if you did not have a polished piece to compare to, so definitely use the polish to get the mirror-like reflection.
Hopefully you can see the difference in the before/after pics. If I had to do this again, I would have just started with the rubbing compound, but I didn't figure it out until I got one side of the car done and wasn't happy with the larger trim piece on the rear. I probably would have saved an hour doing it first, because you don't have to spend nearly as much time working the polish after you've done the rubbing compound. Start to finish, this should only take about 60-90 minutes of your time doing the B, C and D pillar trim pieces. Since I am taking care of the car now, I hope that in the future I should only have to use the polish.
I am not an expert or someone who has a lot of detailing experience, so you shouldn't feel hesitant to do this because you think you might mess something up! Good luck!
