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).

The cover is held in place with a catch at the back which needs to be released in order to lower the cover. Pull it at the back as shown.

These catches get brittle over time and sometimes break easily.

With the catch released, the cover will now lower. You will need to give it a slight tug as it is held in place with a silicone gasket.

The brake switch is located at the front of the pedal box. To release this, twist it through 90 degrees and it will unclip from the mounting bracket.

The switch will lower on it's wiring.

Remove the connector from the back of the switch.

The switch will now be free.

By dismantling the switch it is possible to see the inner workings. By pressing the brake pedal, the switch opens up fully which connects two contacts. Releasing the pedal closes the switch and connects two other contacts. In the following picture the switch is in its fully open position and makes contact with the two upper contacts.

You can see the two lower contacts which are connected when the switch is closed. Common failures occur when the contacts become corroded or damp.

Before fitting the new switch, give the pdeal box a good clean and remove any moisture that may be lurking.

If your brake pedal squeaks, apply some grease to the moving parts. Ask someone to apply the brake pedal and fit the new switch as shown below. The switches have calibration which can be set by releasing the brake pedal once the switch is fitted and pulling the pedal back as far as it will go. This will close the switch as far as it will go.

Take the new cover and refit to the pedal box. The cover comes complete with silicone seal.