The wire to the ECU goes to the Black connector on the loom side.
You want the middle of the 3 rows of pins, pin 19. There may be a white/grey cable there if 2000 onwards.
Connect to this, if not you'll need to connect to the pin.
Would check with croc clip before a permanent connection to make sure.
The other connectors on the back are for 2 for lighting, 1 for switch line power and 1 for earth.
This is for a VDO Tacho