No, I am actually terrible at electronics
I actually asked a friend with the skills to build the cable for me, because I am really awful at soldering and stuff
While I do solder some stuff, I never to try anything like that for my Amiga for fear of screwing something badly, so I prefer to ask friends who know better what they're doing
Yes, the first thing I suspected was the GND line... but the pin is correct in both connectors. Actually everything is correct and I am looking at the connectors from behind (While I am terrible at electronics, I can use a multimeter to test continuity
And I also tested all games on Multiplayer mode, of course. Also using a Master System pad (original one), because I am not even sure a Mega-Drive one will work with this adapter because of lack of 5v+ line, but I see no reason why a Master System one should not work.
EDIT: Ok, I tested it with a Mega-Drive 6 button pad with Dynablaster and the player just moves to a random direction even without touching the controller. With the Master System pad I still get nothing, but with the 6 button Mega-Drive pad, I get a random direction.
That's really weird.