Armor All makes a silicone wipe that reconditions plastic. One of our Fantastic Fans is about five years old, and the polycarbonate lid was looking pretty weathered. When we recently installed a second Fantastic Fan, we took the Armor All wipe that came with the new fan and rubbed it all over the older fan lid. It doesn't look brand new, but it cleaned it up significantly, and you can now see through it.
If that doesn't work, there are two other alternatives: If the lid is in pretty good shape, you can try using some headlight cover plastic polish on it (a few dollars for a little bottle). Or, replacement lids aren't too expensive. You didn't mention whether this is a regular vent or a Fantastic Fan (or other powered vent). The new-Jensen replacement lids cost about $10-15, and I think the Fantastic Fan polycarbonate lid is about $20-25; although, I think I recall that it has a lifetime warranty, which probably only covers breakage and not cosmetics.