This is another reason why everyone who works on any car you own should be trained. A minor failure in a Mercedes-Benz can cause the dashboard's multifunction display to show three simultaneous failures; brake assist, ABS, & stability control. Should the owner pull over and put the car in park, the shifter will lock in place. If the owner has the vehicle flat bedded to an independent brake or transmission shop, the shop may recommend diagnostics to the tune of a couple of hundred dollars. On the other hand, someone with experience will recognize this as a brake light switch failure.
The brake light switch for many Mercedes-Benz vehicles is part number 001-545-64-09. It has a list price of $19.18. If you're a Mercedes-Benz Club of America member, it costs $10.50. The switch can be changed in 5 to 10 minutes by someone who's done it before. It may take twenty minutes for someone who's never seen it before.
To get to the switch, the plastic cover under the dash (above the brake pedal, and below the steering column) must be lowered. It is lowered by removing the four Phillips head screws that hold the plastic cover in place - three across the front edge and one on the left, near the hood release. Next, the vent cover on the transmission tunnel must be removed. The vent cover has a Phillips head screw that should be rotated a quarter turn counter clockwise. At that point, the vent cover can be pulled out, and the plastic cover can be lowered (not removed) from the front.
With the cover lowered, the switch can be accessed. The old switch will look like one in the attached photo. To remove it, rotate the switch a quarter turn clockwise, and pull it back. The switch on a 2002 CLK has two electrical plugs with three wires on each one. After noting which side of the switch each electrical plug is plugged in on, the plugs from the old switch can be transferred to the new one.
Installation is the reverse of removal.