There are quite a few threads that have great info about the BAS/ESP Light that helped me out a ton when I had the issue a while back, but I didn't run across a thread that actually showed how to replace the switch since that is one of the primary culprits of the dreaded BAS/ESP light. It isn't very complicated, but I know there are people like me that like to see what we are getting into in advance and put off the work prior to having details. Besides, so many of the more complicated service tasks have been well documented with pictures by other members, I have to grab opportunities to make a contribution to the forum!
For many projects on the SLK, it helps to have a bathroom mirror like in the picture to minimize contortionist activities. The tiny extension-rod mirrors are great for certain tasks in cramped quarters, but often a wide angle view is golden. Your significant other will be thrilled with you... so buy one for the garage to get scraped up and greasy over time.
The general location of the brake light switch can be seen in these photos, it is on the shaft in the footwell that the brake pedal pushes against. The switch is the black plastic part between the red wires and the silver metal brace.
Here is a mug shot of the b!@#*d:
1. Disconnect the negative battery cable. Have stereo code available for after reconnecting the battery.
2. While it helps with access, the hassle and potential broken aging plastic tabs and paint scrapes should lead you to be a contortionist and skip removing the lower instrument panel cover.
The part is just above and shielded by the lower plastic instrument panel cover, making this a contortionist exercise to replace. Wear long sleeves, you can see the blood on the panel in the picture above! (ok, just the original red paint bleeding through before I re-repainted the panels)
3. Disconnect the brake lamp switch harness connectors (1).
4. Press the locking tab (2) and rotate the brake lamp switch
(4) clockwise. (Only quarter turn or so if I remember correctly)
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 the 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.
The most entertaining post I've run across on benzworld related to the BAS/ESP light switch, you should read this to offset the irritation of scraping your wrists on the lower panel performing the above procedure:
Hilarious Benzworld Member Post
Happy Twisty Roads!