I had Mercedes replace my back Left Rear Light 2-3 months ago. I cracked it backing up but it worked fine.

Anyway they installed MY back left rear tail lamp assembly (My PART I bought on eBay)

Worked fine and no issues.

Today my neighbor came over and said your lights are on? It was strange so I started car and then shut off and locked my Car!

When I pressed Key Fob to Lock, The Right Blinked 3 times and then stated illuminated.

The back Left was NOT lit at all?

I read someone said something about a Sensor being near trip computer?

One guy with same issue said YES, he also installed his rear left light assembly with used part and he said when he installed it, he found a broken Solder joint in the ribbon wire going to “that little silver sensor” that he must have knocked off.

He fixed the broken solder joint and it worked.

My warranty is up Except they’re going to fix my steering wheel that is a lot of board for free, because it happened when I brought it in for my last Warranty appointment last week.

 

Could it be you accidentally turn on your rear emergency light?

 

I will check. I didn't know there was a rear emergency light? Not exactly sure what you are referring to? But I appreciate the help!