What do you suggest as the best way to tint the windows but they stay see-through?

depends, is it the model kit? if so i suggest strips of transparent paper (used mostly in binders for colour coded filing) then carefully appplying it with a light paste around the windows. for more referance on this vary topic please goto my site and click on “past specials” in there you can access “the ultimate ecto-1a model” and you'll find evrything you need to know there.
im gone.

Finally a post that belongs in the props discussion..

I don't know if you are refering to a real car or a model.. for the model.. you can buy window tint for R/C Cars that should work.. or you can just paint the windows a smoke grey..

For real cars, they sell window tint kits, but they have to be custom made.. and I believe the 59 caddys had them come with tinted windows anyways..

Dont forget that the windows on the Ecto-1A were only tinted on the top and bottom, so put a strip of scotch or masking tape in the center of the window to make it look more “movie” like.