Hey, Willie, welcome to the forum.
I'm guessing you didn't get a reply in the 203 forum since no one else has had this happen. It certainly doesn't strike a chord with me.
Couple of thoughts come to mind, though.
First, are you the original owner? If not, it's possible there was an accident that damaged the left rear quarter (that it's only left bulbs affected suggests that possiblity) and that although the cosmetic damage was repaired, there is a squished channel where wires have worn through creating a short, etc.
Second, if the 203 is like the 210, then you have essentially printed circuits for the lamp sockets and a little plastic base with contacts into which the bulb fits, then that rotates into the light opening, right? If so those contacts are a finicky setup in the 210, perhaps you have the issue in the 203. There are a few posts in 210 about that issue.
It's also possible a connector to the light assembly has failed or corroded, check those wires well.
The external fuse thing is odd. It's entirely possible that part of MB's "knowledge sharing" with Chrysler resulted in some -- shall we say -- interesting engineering, so perhaps there is actually an electronic module that controls all your external lights. And if so, it's possible some part of that has failed...I'm sure it would cheap and easy to replace...NOT.
Anyway, that's all that comes to mind. If none of the above makes sense or pans out, you can also try calling a couple of dealers. If there is a TSB or recall they'll let you know, and often the SAs (if bored) will chat with you on the phone and give you a few ideas.
Last thought: once you have your fix, please post a synopsis in the 203 forum so the next unfortunate soul with the issue doesn't have to endure my rambling.
Take care and enjoy the ride,