First thing is usually to try mild soap. A cap of Woolite in water on a dampened rag. If there are a lot of smoke or tar stains, isopropyl alcohol won't attach the plastic. I would just strip the shell of hardware before getting into a really vigorous cleaning. Also dampened rags or microfiber cloths. Don't soak the shell with anything. Remember, the idea is to remover the gunk. Not smear it around. That means lots of rags and constantly changing the part you're rubbing on the shell.
Then there are the polishing kits for restoring headlamp lenses that have superfine foam backed abrasive pads and polishes.