Found this on the BENZWORLD FORUM. Thought it might be of interest, and help someone who is considering doing it themselves. I made some minor additions to the original post to clarify some of the steps, but is pretty much as originally posted. Hope it helps!
REMOVAL AND REPLACEMENT OF THE BRAKE LIGHT SWITCH – SLK R170 SERIES
1. Disconnect the negative battery cable
. Have stereo code available for after reconnecting the battery. (There is some question as to the necessity for this as there is no power to the “BLS” with the ignition switch turned out of the “run” position, or with the key removed from the ignition.) This is part of the procedure in the MB maintenance manual.
2. While it helps with access, it isn’t necessary, and the hassle and potential of broken aging plastic tabs and paint scrapes should lead you to skip removing the upper instrument panel cover. Just remove the “black plastic” lower instrument panel
cover by removing two screws, and CAREFULLY removing it from the left side kick plate. You will have to lift the panel up considerably to get it over and off of the tab at the top of the kick plate, and most likely will hear a loud “snap” like breaking plastic as you do. This is normal.
The “BLS” part is just above and shielded by the lower plastic instrument panel cover, making this a contortionist exercise to replace. Wear long sleeves.
3. Disconnect the brake lamp switch harness connectors (1).
4. Press the locking tab (2) and rotate the brake lamp switch
. (Only Ľ turn is required.)
5. Remove the brake lamp switch (4) from the brake pedal brace (3).
1. When you get the new brake light switch, don't press the white plastic part into the black body, or if you do, you can pull it back out. The length will be set as you install the switch and the first time you press the brake pedal after installation, and a wild guess on my part you may as well press the brake pedal for the first time with the engine running / power brakes pressurized to get the 'normal' brake pedal travel.
2. Install the brake lamp switch into th brake pedal brace (2) and rotate counterclockwise until it hits the stop. If I recall, you may need to hold the brake pedal out a bit against the force of the spring while initially seating the brake switch.
3. Connect the brake lamp switch harness connector (1).
4. Connect the negative battery cable.
5. Perform the BAS/ESP Reset Procedure - with the engine running and the steering wheel centered, turn the wheel all the way to the Left lock
, then all the way to the Right lock
, then back to center. Note this procedure relates to the battery disconnection, not because the brake switch was replaced.
6. Enter your radio code.
7. Hope your problem is solved and you do not need to continue trouble-shooting.