Hi, yes, I checked the gound by adding a ground wire manuall to see if that was the issue but the problem persisted.
I have the socket, so I put a bulb in there to test. but forgot to mention that I have digilights installed, not a bulb anymore (original bulb socket is till hanging there). The wiring on both of the digilights is soldered to the same lines, I checked that and resoldered them even, including ground, just to make sure all connections had good connection.
The running light works fine, as does also the turn signal in the passenger side. Only when I push the brake it does not light up. On the other side all works fine (running + brake + turn)