Have you figured out what scenario causes the indicator to light up? Is it on all of the time, or just when you tap the brakes for instance? Of does it come on, but turn off when you switch on the headlights (or vice versa)?

Your courtesy lights are not part of the system, just the exterior lights. And as DJNEWK2 stated, the license plate bulbs are commonly overlooked.

Also, a bulb on its way out will set off the dash light, even though it still appears to work. Look for any bulbs (especially in the taillight housing) that appear discolored. The system works on resistance, so a filament on its way to burning out will give off a different resistance, and possibly set off the light.

Note that when replacing bulbs, the wattage and housing is very important. For the taillights, the correct bulb wattage is embossed near each respective socket. Use Philips, Osram, or Sylvania, with the nickel-plated housings, not the domestic cadmium-plated ones...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car