Late 156 had one screw (turnable my hand) added because people kept breaking the clips.The socket is def behind the panel to the right of the steering wheel - small cmpartment, undo screws and above fuse box.
I have a 156 (2005) and it is there on mine.
Suppose we should ask what diagnostic he's hoping to use .. if it's not MultiECUScan or AlfaDiag, he's going to be out of luck on a 156!
"FiatECUScan" is just the old name for "MultiECUScan" (they changed the name when they started to cover a couple of Suzuki).Why's that? I bridged wires on my ELM327 El'cheapo and I am using FiatEcuScan on 156, 147 145 etc without issues.