A drive mechanism assembly for enabling driving and manual movement of a sliding side door. The assembly has a track assembly mounted to a vehicle body at a predetermined height and a drive mechanism mounted to the sliding side door so as to be proximate to a door latching mechanism. The drive mechanism has an input drive motor driving an output gear assembly engaging the track assembly and a transmission gear assembly with a sliding gear which selectively couples the input drive motor and an input gear assembly to the output gear assembly between an engaged position for driving movement or disengaged position for manual movement of the sliding side door. In this way, the slide side door may be moved manually between open and closed positions without having to overcome the effort required of rotating the drive motor.