This is a common component to fail on both the fortwo and roadster, the obvious symptoms being a lack of brake lights and gear selection issues.

Jack the front of the car up and secure both sides with axle stands.

Drop the front undertray by removing the 8mm securing bolts.

Now you will have access to the bottom of the pedal box (shown below).

Remove the back securing clip and gently lever the cover from the master cylinder end. It will come free fairly easily as its only held on with a silicone gasket.

With the cover removed, locate the brake light switch.

Twist the switch half a turn either way to release.

Finally unclip it from its holder and wiring loom.

Refit the new unit in reverse of removal.