Here is a brief description of how to fix a very common failure on Mercedes W126 chassis vehicles. If you have an exterior light that will not come on for some reason, even when you've already replaced the bulb, here is the first place you'll want to look after the fuses. The solder joints attaching the circuit boards to the plug terminals crack from vibration and thermal expansion/contraction. Getting a good hot soldering iron and some fresh solder on those joints will revive these modules 90% of the time! Go ahead and re-touch ALL of the solder joints connecting the board to the terminals. Also, verify that the large metal resistors (they sort of look like thin heat sinks) aren't burned up or otherwise damaged. Doing these steps usually fixes any dead light sockets, and clears up any issues with the "bad bulb" warning light on the dashboard.