Simple Green hose it down soak and rinse. Follow up with Chemical Guys heavy duty cleaner spray and wipe down. Just don't spray directly into the alternator or into any electrical connection and you will be fine. Clean mine every few months.
Try the Chemical Guys cleaner Ian, I haven't put any dressing on plastics, it cleans it so well it looks treated. I did my mom's car a few months ago it even got rid of the nasty buildup on the air intake on her Accord. The old man swore I put dressing on everything.