It seems obvious to me that your door switch is faulty. There should be a little black thing coming out from the B pillar when your door is open. It must be faulty...
Sorry, but I don't know the repair on this one? I assume you must remove the interior trim on that pillar (which is much easier than it looks) to inspect it even further. Replacement should be relatively easy?

Again, good luck!

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)