The Roadster comes with a button to turn off the ESP but the fortwo never did as it was deemed to be less stable. However, the wiring between the fortwo G2 (2003 onwards) and Roadster are nearly the same. So, with a bit of tracing, we discovered how we can replicate the same feature that somes with the Roadster. Other methods out there permanently disable the ESP until the car is restarted, but in reality this is not really a sensible option for a saftey feature such as this.
There are 3 connections required to obtain this function, tracing them has been quite difficult, as the main ESP wire does not go directly into the SAM unit.
There are 5 connections on the back of the switch: the centre one is the main earth, the two end connectors are the illumination circuits, and the inner two are the function connectors.
The smart switches are generally two switches in one block, in this case the ESP function only utilises one side of the switch, so uses the common earth, a single illumination and the ESP signal wire.
The illumination is taken from pin 7 on Socket 5 on the SAM unit, this is the same illumination circuit as the Roadster pods.
For the earth we again share with the main earthing point on the passenger wheel arch (also like the roadster pods).
Finally, the most important wire is located in the middle of the main wiring harness which comes from the ESP unit on the underside of the car as it enters the cabin. The harness must be carefully cut open, this reveals a mass of wires, in the middle of this bundle is a Blue and Yellow wire, this needs to be located and carefully spliced into.
For internal applications, the use of scotch locks for low current wiring is perfectly fine, and makes the process much easier. The wiring connections from the switch are as follows:
Centre Pin = Brown (0.75A) --> to chassis earth
Top Pin = Brown/White (1.0A) --> pin 7 socket 5 on SAM unit
Signal Pin = Blue/Yellow (0.75A) --> Blue/Yellow (Main harness from ESP unit)
The ESP switch only utilises one 'half' of this switch, so uses the common earth, a single illumination and the ESP signal wire.
Finally create a suitable mount for the switch, we chose a subtle location for the switch, mounted in the under-stereo drawer.