Have you taken a look at silkolene pro-prep? I think silkolene calls it a hard surface protector - or something like that...
Roostafish here recommended it to me. Works great. It is best described as industrial strength pledge on steroids. It is silicone based and has a consistency that covers scratches and shines up the surface. It is great for keeping off mud and dust too - two thumbs up here. I spray it under the fenders too, while taking care to not have any overspray onto the brake rotors.
Description of the product from the mfg is...
• A high quality, hard surface conditioner
• Pro-Prep enhances the color and finish of plastics and painted surfaces and inhibits the further adhesion of dirt and mud while protecting
jeff
p.s. To get plastic clean - I use S-100 or honda-brite (honda labeled s-100), it removes clay stains, etc. By the way, both s-100 and silkolene should be available in europe - I believe that is where they are made...