the regulator is a mechanical piece. it is a metal bracket with slides and a pulley on it that converts the spinning of the window motor to an up and down motion that raises and lowers the window. if you are mechanically inclined in the least, it is not a hard job to do. the only special tool you'll need is a rivet gun, as the regulators on MB's are riveted into the door supports. i'm pretty sure the door panel is easy to remove on a 124, but as i'm not totally sure about the process, i will find this info out and post it later on today.