I tried this a while back and had a little luck using a white background like a piece of paper over the slide and just using my  (now archaic) mustek scanner. In most cases though it was still too dark. I know that they make (or used to) a scanner for slides but I don't have any current information on this. 

( Use the order: scanner slide paper - well I know you wouldn't but  some might get 'em backwards)