I had no brakelights so my mechanic replaced the switch located below the master cylinder and then I had brake lights. He also changed the brake fluid and flushed the lines since the car already had about 93K and was due anyway. Shortly thereafter the BAS/ESP light started to come on after driving for about 10 minutes or thereabouts, depending on how much I used the brakes. Of course, when the light comes on, if you place the car in park with the motor running it won't come out again unless you re-start the engine (or overide with a pencil at the back of the shifter). I looked in the book and tried the remedy of turning the wheels lock to lock - no luck.
Do any of you have any suggestions? Any chance changing the switch or the fluid could have played a role? Should I bite the bullet and take it to the MB dealer and see what the code is?

 

I'm not sure which switch is "below the master cylinder." I would be inclined to get a new brake lamp switch for that issue. They are notoriously bad in that generation ML, and are relatively inexpensive. If this is what your mechanic already replaced, get him to warranty it and make sure you ask for either OEM or first supplier quality.
If you want to do it yourself it's really easy. Take down the black plastic panel above the pedals. The switch has 2 plugs on it and is attached at the brake pedal. Make sure that the plunger on the new switch is pulled all the way to the outward stop before installing, push the brake pedal down during install, then pull the pedal up as far as you can.

I would get the codes read first though. That truck is OBDII compliant, so your local auto parts store should be able to read them out for you.

 

Thank you sir!! I will give that a try!!