I had them in for over a year and they stayed clean enough on the inside. Maybe a little film but nothing really noticeable.
Someone asked earlier about colors. If you go to Michaels art/craft store, they sell cans of Krylon that is made for coloring glass and its around $10. Just put a number of coats on it, lightly, to get them colored. They have red and yellow usually.
Did this on my forester and it looks great. The light comes out very yellow as well, no need for colored bulbs....
