They usually like to sign the actual translate, but they won't open it for you. If you can remove the edge pieces and then pull back the translight, they will sign directly on it. If you cannot get the translight exposed or you are worried about damaging it, just have them sign on the glass. If it wasn't a translight and was in fact a silkscreened back glass, they would sign on the glass anyway.. If they are called upon to sign often, then they usually have special pens designed for glass. I have had them done both ways.