License plate light diagnosis story:
I'm ... how to put this ... barely competent at wiring. It's never been a strong suit of mine but I'm trying to get better at it. That said, a vintage German car should give me ample opportunities to learn. One such instance is the license plate lights.
When I got the car they didn't work. I asked the PO about this and he said he "never looked into it", which was a funny statement coming from an otherwise very meticulous guy. Fine, maybe it wasn't a priority for him, or perhaps the problem was deeper than burnt out bulbs.
Some probing with the test light/multi-meter found 12v to each one of the light sockets, so bulbs were ordered. Installed and nothing, not surprising really the bulbs in there didn't look bad. So, trunk was closed and problem ignored for the rest of the week.
I went out to the garage one evening after putting the car "away" for winter and just did some general garage organization and decided to look at the license plate lights again. Seemingly had power, but some further investigation it seems like I didn't have any ground. Common enough issue, and I'm no stranger to the sentiment of checking and cleaning grounds especially on old or rusty cars.
So, I trace the grounds back to the trunk where there's a junction of about 7 brown ground wires from various trunk components on 2 ring terminals. No wires appear loose at the terminals, and the body isn't rusty where the ground is attached. I disconnect it, sand the paint down, and reconnect it anyway. Presto! License plate lights!
I close the trunk lid and start cleaning up my tools feeling good about myself. Before I go in I flip the lights back on to bask in the glory of a well-lit license plate. Ooops, lights no longer work. Checked my voltage at the sockets again and now it seems like I have power on the ground side of the socket. That doesn't make much sense, but again I'm borderline incompetant at wiring. I start googling and trying to find a wiring diagram, maybe I'm probing the wrong side of the socket, what do I know.
I find a few posts about how the wires for these lights like to break up by the hinge where they flex throughout the years. I turn the headlights back on, and grab the harness by the hinge and flex it around, lo and behold the license plate light flickers. So, we must have some broken wires there. Now, this is all loomed up very nicely, tucked into a hollow part of the hinge and behind the carpeting of the trunk. Interestingly enough it's the crinkle loom you see everywhere, but Mercedes has the audacity to use a fully closed loom, not the split style you get at the parts store. So after some careful cutting with a razor blade to not damage the wires hiding inside, the loom is split and I these 3 wires jump out at me:
That'll do it! So, soldered, heat shrinked, and re-loomed and we have license plate lights! And as an added bonus we now have an interior trunk light I didn't even know was there.
This was a fun, free fix, and is something I'm proud I bothered to fix. I know 5 years ago me would have laughed it off as just another example of why German cars are crappy and never taken the time to figure it out.