Is the 12V socket the only thing that's missing? Do you have the double/false floor, elastic band on the left side and net on the right?
M135i with an internal M Performance package (7ML) has the trunk accessories limited to bare minimum and the 12V socket is one of the things taken away.
Maybe your M135i was specced with 7ML?
Pictured above is the comparison between full trunk setup (top - without 7ML) and limited one (bottom - with 7ML).
There is also a third type of setup (it's basically 493 option without now missing 418):
I happen to have the picture of right hand side wiring of my car (LHD, made 11/2021, without 12V in the trunk):
View post on imgur.com
As far as I was able to tell - there is no plug&play option for 12V port nor for the extra light on LHS.
I did the trunk retrofit to have all the cool stuff, but skipped electrical part - didn't want to touch it in any way to avoid risking warranty issues.