polish it. Get some good metal polish and a lint free cloth like a 3M/scotch brite High performance cleaning cloth and polish it up. It will take alot of elboe grease, but it will shine up nicely. Or you could get a bullet shaped buff for your drill and do it that way with some polish or rouge.
Trouble with those grilles is that they were made of thin annodized aluminum and were never very shiny even on the show room floor. Best description for that finish is 'bright' but definitely not shiny. If you are moer low budget than great looks, use HKs polish routine (Dupont #7 rubbing compound for the rough work followed by Mother's mag polish and TONS of elbow grease will give you a mirror finish). Problem will be that you must wear off the annodize which is very tough. Actually it is aluminum oxide which is what they use to make grinding stones! When new, annodize is like a clear coat. As time goes by, it absorbs moisture and the clear coating clouds which loses the bright finish. Just polishing the annodize won't eliminate the cluody looks which is why it looks dull. Once that is off the base aluminum will shine right up but will require a lot of follow up care 'cause it won't have the protective anodize coating. Keep a good coat of car wax on it at all times.
If you are doing a 100 point show car, you can have the grille chrome plated which will be very attractive and low maintenance. I guess a chrome job will be in the $100 range. Cost will be higher but effort on your part will be low and results will be striking.
Remove the anodize by using oven cleaner. This will take it off. After that, polish it up. There is a new product out now called Zoop Seal. It is for aluminum and it seals and protects. Rather expensive but from what I read, well worth it.
Thanx for the tips Kevin. Since the oven cleaner is caustic, doeen't it also attack the base aluminum just as aggeressively. In fact the only way I can immagine it working is by dissolving the base aluminum, loosening alumina coating. Alsl what is the Zoop seal like? Is it a liquid , paste, ? Where do you find it? How expensive is expensive?