A modular trim panel unit for the manufacturing and assembling of doors for motor vehicles, with the preassembly of the interior trim panel for the door including one or more basic mechanical and/or electrical components, which unit is manufactured off the main automotive assembly line, with the preassembled modular trim panel unit to be brought to the main assembly line for final assembly of the door. The trim panel unit adds no significant structural reinforcement to that of the final door assembly and is made up of at least one molded layer and includes the drive and motor of the window lift mechanism, with the window and guide(s) therefor and a drive connection being mounted on the main door frame structure. The panel unit can also include parts, if not all, of the door latch mechanism and door lock motor, as well as the actuating mechanisms therefor, and an electronics module (e.g., a remote multiplexing unit), all of the door switches, and preferably most of the lateral extent of the electrical wiring for the various electrical door components, all completely mounted and interconnected. The innermost molded layer can include a series of cavities or support and positioning walls for holding the various mechanical and electrical components in place, including a covered "box" area into which uncovered circuit board(s) can be inserted, as well as straddling plateau areas for making screw connections for the components.