brush the ac vent with some shampoo and a tooth brush. keep brushing for a minute, then remove the residue with a damp cloth, rub it with a dry cloth to dry it, then spray any good interior polish like formula, if it goes away ur problem is solved, if it doesnt get ur hands on an ear bud/ q tip, apply some engine oil to the q tip and oil the vents with this, leave it for a bunch of hours and then removed it with a cloth. after it is clean and dry again apply a plastic polish, i used this treatment on the whitened parts of my car