I just read a tip from a paint expert that linseed oil and paint thinner mixed 60/40 works best.
Plastic can loose its colour and look faded after a few years of usage. Whether it be plastic windows, exteriors, cars, motor bikes etc. There are a number of products available in a lot of car shops, home improvement stores and online that you can buy and use on plastic to restore its colour and quality. I would recommend carnauba car polish and cutting car polish. These products are exceptional and unique. Use need to follow instructions on the product label and apply it only on a dry and clean surface using a sponge or a dry cloth. Then there are other plastic sprays available in many of the car stores. But you can use them on any home objects or bikes etc.