Originally posted by andy_k
you need to isolate which bulb is setting the warning off.....
1, does it come on when you turn on the lights and go off again when you switch them off? - if so a check of all the bulbs will usually show one that is either not working or dimmer than the others - nearside tail light seems to be a favourite for causing this problem, although the bulb is lit it's not got a clean connection - remedy, clean the contacts and replace both rear bulbs.
2, does it come on when you put the car in revers then go out again when you put it in drive? - if so one or both the reversing lamps are out.
3, and most likely, does it come on when you apply the brakes and then not go out again until you restart the car? - if so one of the brake lights isn't working. Replace them both and don't buy the bulbs from Halfords as a few on here will vouch for, they seem to make their bulb sleeves ever so slightly smaller than everyone else so they never make a proper contact in the bulb holders
The bulb warning circuit on Mercs is incredible sensitive so the slightest bit of dirt, corrosion etc on the terminals can cause problems.
HTH
Andy