Date registered: Mar 2009
Vehicle: 2006 E350
Location: Phoenix, AZ
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Mine was caused by an intermittent floating ground
I experienced the same problem a few years ago on my 93 300E (W124).
One day I noticed that when I pressed on the brake pedal SEVERAL bulbs in the taillight assembly were coming on (not just the brake light bulb). I traced that to a bad ground. The ground side of all the bulbs were connected together with metal strips and in one spot all the strips were not making a good connection with each other due to age.
This problem had clearly plagued the previous owner for some time because when I acquired the car I noticed that the 'Bulb Out' indicator bulb in the dash had been removed. And because it was the brake light, guess what? Yup, the cruise control was also intermittently inoperative.