I just soaked a Q-tip (one of the long ones hobby stores carry) in WD40, then wiped it over the louvers on the vents. That caked on, baked on dusty film wiped right off. I tried other cleaners with no luck. This takes it right down to the plastic. Now they look brand new and black and clean and shiney. It also lubricates the mechanism so the vents move freely as well.
