I wrote one to these pages over a year ago. I have been doing it since the 500SELs started coming with 2nd gear starts years ago.
It really isn't that big of a secret as MB did the same thing on 78-80 116 chassis 300SDs.
The system will require two relays, and oil pressure switch and some wiring.The concept is to use one relay to electrically energize the kickdown solenoid when the brake pedal is released: brake pedal - no kickdown, no brake pedal - kickdown. The oil pressure switch is placed in the governor pressure test port. The switch will open with 5psi. The governor starts pressure at 10kph so the switch will stay engaged till then. The circuit is a little hard to explain in words other than to say that one relay controls the other. One relay will have to be an either or relay (one that always conducts either to 87 or 87a).
Operation is like this. At a stop with foot on brake, car is in 2nd gear. As foot is removed from brake transmission is kickdown to first (imperceptable), foot presses the throttle and car accelerates in firstgear. If full acceleration is required smooth pedal to the kickdown before 6.2mph will keep the car in kickdown and all further shifting is determined by the cars controls as the system we have provided shut-off at 6.2mph. If casual acceleration is applied the car shifts from first at 6.2mph and assumes all the normal characteristics.
Bosch Master, ASE Master, L1
33 years MB technician