How Do I Know If The Brake Light Switch Needs Replacing?
If you have problems selecting reverse or getting from reverse to neutral or indeed if you
notice none of the brake lights are working, chances are you will need to replace the switch.
The switch is 2 separate switches in 1 package. 1 switch controls the lights, 1 switch controls the traction
controller. 1 of these switches can fail separately to the other so the brake lights could function ok but
the electronics side could have failed which will bring on the ESP/Trust+ and ABS warning lights
The brake light switch failure was one of the first common faults found on the fortwo. This fault rarely appeared
on the Roadster even though they used exactly the same brake light switch, why? It's all about water.
The information you read on other (copycat) websites about replacing the switch is wrong.
They don't take into account the switch calibration.
Why Has It Happened?
The failure of the switch is almost certainly down to 1 of 3 possibilities.
Switch contacts failed.
Switch short circuited due to water ingress.
Switch submerged in water.
The switch box cover has a drain port which can get blocked,
massage this port to see if any water comes out.