It could be the 'long' relay inside the fuse box. Also try this

1) remove the light bulbs in pair, tail lights, stop lights, low/hi beam, parking etc. Start the engine and the 'light bulb failure light' should go off at some point with each removal. The last pair you removed would be the problem. I think you need to press the brake for the stop light.
2) If you remove all the lights and it still stays on then it could be the relay or wiring.

Hope this help.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

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