Hi Ninian,
I do not think that I can be of much help in locating connector D004.
Grasping at straws, there is a similar 2 pole connector on my x244, which carries D+ and oil pressure (both being signals from the rear of the engine). This connector is at the RH rear of my engine bay, tucked behind the battery. No battery there in X250, but as your D004 also carries the D+, could be in similar location.
My other suggestion is that on the x244, connector D004 is a larger multi-pole device. It is buried IN the engine bay fusebox, below the fuses and relays. Given what is shown on the eLearn diagram, it could be worth checking, but take care not to short anything, or you could blow the midi fuse at the battery.
When I suffered from starter problems in the past, there was too much resistance in the solenoid circuit. I got a clunk as the pinion was engaged with the ring gear, but there was insufficient power to allow the solenoid to complete its stroke and close the motor contacts.
I suspect that when your emergency technician applied a jump start connection at the starter motor, he just connected to the smaller solenoid terminal. I think that it could be more difficult to connect to the main battery terminal on the starter, and then energise the solenoid.