Many here like Nevr Dull and Simichrome. My local electronics repair guy likes Flitz metal polish better. He just demonstrated it, and it works well. He rubbed some on my 24 year old plastic Bleumels fenders, and the patch he polished looks like new. I didn't think there was hope of cleaning them.

Then I got to thinking. This stuff is expensive. Can I make my own? Well, yes, and I can do it non-toxicly (sp?), too. Check out this article! Scroll down to where it gives recipes for "metal cleaners and polishes".

If it works, that's great, but be aware that there could be substances (Oils and solvents) in some polishes that might not be good for plastic and can lead to eventual (usually does not happen instantly) cracking and degeneration. you have to make sure that the product is "plastic friendly", before using it to clean plastic parts on your bike. An email or call to the manufacturer can usually get you that information.
Also, be aware that many aluminum parts have either clear coating or clear anodizing that might get rubbed off completely if you are using certain polishing compunds. Once these clear protective coatings are gone, the aluminum will be exposed to the elements and will be very vunerable to dulling, oxidation and corroision if you do not religiously clean/re-polish them.
I always try to use the mildest least abarasive cleaner/polish I could use on my bicycle and motorcycle.....Pledge furniture polish has kept them nice and clean all these years! Got the idea to use it from car show detailers. It's harmless to plastics too!

Chombi
84 Peugeot PSV

I've use Brasso on plastic tail lights with great success.

I'm a recovering car nut.

Blue Magic, at times.

Meguiar's makes a plastic cleaner and a plastic polish. They work very well on plastic lenses. I suspect they would work well on plastic fenders the OP mentioned.