The electric trunk divider was not working on my 2018 A238 E400 Cabriolet. You could move it manually up & down without it binding so I assumed the internal motor and gears were still good.

At first I looked at the red push button on the inner trunk lid. It lit up but tested intermittent at the switch leads. It looked a bit corroded so I disassembled it and cleaned the internal contacts. There are some very small parts inside that switch. I'd do not recommend taking it apart. Just soak it with some D5 switch cleaner or get a new one. The no electric up/ down divider problem still persisted.

I understand that the wiring harness in the rubber loom on the left side of the truck lid could be suspect as it flexs every time the trunk lid is opened and others have commented on finding broken wires inside that.

I had the left inner trunk cover pulled back to look at the back of the taillight when I checked the lower end of that rubber loomed trunk lid wire harness. Sure enough, lots of green & white corrosion was visible inside and one of the pins were broke off. I cleaned the connector ends with D5 and bypassed the broken pin wire in the connectors with an external 24 gauge wire. That fixed the electric divider so now it operates with the push button properly !

 

Good fix. Those harnesses in the left trunk lid rubber boot are a known weak point on the A238. Flexing every time you open and close eventually snaps wires inside the insulation. The corrosion you saw is typical once moisture creeps into cracked insulation.
Your bypass with the 24 gauge wire is exactly the right approach, no need to replace the whole harness for one broken wire. And good tip on that switch, tiny parts inside, not worth the headache. D5 soak or replacement is the way.