Whenever I start the car, I immediately get 2 warnings:
-- ESP malfunction visit workshop
-- BAS malfunction visit workshop
When I try to use cruise control I get a message about speedtronic malfunction.
Right before I started getting these warnings, there was a weird thing that happened with my window. It started raining so I closed the roof. I opened the windows and then closed them while accelerating. Driver side window closed normally but passenger side stopped half way and I am pretty sure I heard a beep from the car when this happened. I had to press the window switch again and the window closed. I have a suspicion that the battery voltage dropped when the window failed to close. As soon as that happened, I noticed the ESP/BAS malfunction warning.
ESP seems to be working as always. That is, the car seems to correct for oversteer and the yellow triangle flashes as usual.
Other than the messages and cruise control not working, I detect nothing wrong.
I have tried calibrating my steering angle sensor by turning wheels all the way to the left and then all the way to the right.
After having read about this problem on the Net, today I removed the plastic panel below the steering wheel to see if I can take a look at the brake light switch.
There is a plastic shroud that prevents me from getting access to clutch and brake pedals. I tried to remove it but could not. I did not want to break it, so I was not very forceful. Can anyone tell me how to remove this plastic shroud?
I have done this on several other cars that needed brake pedal adjusted or brake light switch fixed or replaced. However, for the life of me I am not able to figure out where the brake light switch is on my R171.
I can see the clutch switch.
There was also another switch that sits on top of the plastic shroud. I was not able to feel its plunger from inside the plastic shroud with my finger. I am not sure what the switch is. It is part #0045452114. I was able to remove it by rotating. I tried pressing on the plunger to check if the brake lights will turn on. They did not turn on. When I pressed on the brake pedal with this switch removed the lights did turn on. I tested this switch with an ohmmeter and it does seem to respond when I press the plunger. Open with the plunger out and closed with the plunger depressed.
I could not locate any other switch on the brake pedal. I am beginning to suspect that it may be inside the brake booster on the other side of the firewall.
Does anyone know where the "other" brake light switch is located? The one that turns on the stop lights?
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Honestly, the easiest thing to do would be to read the codes on the car and see what's causing the errors. Brake lights are controlled by the rear SAM, and are triggered by a CAN-C signal if my memory serves (I'm on the road, with no access to WIS...).
There are *very* few physical switches in the R171 that do anything other than generate a CAN signal, so unless you have a physical failure of a switch, it's easier to ask the car what it thinks is wrong and work from there.
My dodgy Bogota hotel intertubes also time out trying to connect to TIPS or EPC, so I'm having to work from memory! Luckily, I'm headed back towards London at the arse crack of dawn tomorrow, so I'll see what I can dig up Sunday afternoon when I get home!
In fact, I was thinking along the same lines and began by searching for a way to read the trouble codes. However, unlike VAGCOM, BMW GT1 and other systems I am familiar with, I have not been able to find anything remotely affordable for a shadetree mechanic. The best I have been able to find so far is some Taiwanese set for $5800 on eBay.
I forgot to mention that I get solid 12.5 V with the engine off and 14.5V with it on.
OK so the switch you had out is the clutch switch, the brake lights are triggered by a sensor inside the booster that measures diaphram travel, the signal travels as John says (Sensor>ESP CM>CAN-C>CGW>CAN-B>R SAM>LED).
It sounds like either an internal control unit fault or a failure in BAS to me, as John rightly says, your best bet is to find out what the car thinks is wrong and then determine what *is* wrong based on that, dealer shouldn't charge more than one unit for the diagnosis and then you can handle the fix yourself.
Spending money on the kit is a huge investment unless you plan on staying with Benz for a VERRRRY long time!
OH while i think about it, why dont you go to the Startekinfo site that MBUSA maintain and look into the passthrough system that they are legally required to offer, that will be cheaper/less functional than proper DAS but will allow you to read all codes.
ESP BAS speedtronic malfunction 
