Penetrol, pour on a rag and wipe into the plastic. It's an oil paint conditioner. As the plastic dries out, it becomes faded and brittle. This stuff renews the plastic, just like it does for oil paint. One application will last two to three years depending on the direct sunlight it receives. A can costs $10 at Home Depot, in the paint dept. Used to use it on my old Wranglers, it worked great. There was a thread on this subject a few weeks back.