I am working on a 1939 Tudor. I have removed the door window and replaced the window channels and felts. Looking for advice on replacing the window. Must the window be installed with the regulator in place, or can the window be installed on the regulator at the bottom of the door and then move the assembly (window and regulator) up into place? If the window must be installed with the regulator in place, any tips for installation of the window into the regulator channel?