Carraige windows were referred to as "glasses".

Look at this from The Footman’s Directory and Butlers Remembrancer (1823), by Thomas Consett, from the chapter "Going out Visiting":

If you have to go out with a carriage, as soon as it comes to the door put the blinds down, and the glass father up: but if it rains , do not put the blinds down till the family are just going to get in; when you put down the glasses, or put up the blinds of the carriages do not get into it with your dirty shoes, as the coachmen have enough to do without cleaning after you; if you cannot do it without , let down the steps and kneel on the edge of the sill of the carriage where the door shuts; if the carriage be one which opens, you must be very careful that you do not attempt to do it when the glasses are up, for if you do, you will most likely break them, which will be of serious consequence, as they are very expensive.