WIKI

Connection to this TCU can be established with the ZN151 DS Box and the CB403 - DS-BOX Extended Cable Set for direct connection

The connector of the FDCT does not have a PIN for GND, so you have to connect the GND black connector cable anywhere on the metal body of the TCU (see picture below)

To establish the proper connections follow the steps:

VGS2-FDCT Mercedes
VGS2-FDCT Mercedes