Factory VDO Oil Temp Gauge: Centre Consul

Changes over looms for one that was burnt out in a 1983 tin top.  The replacement loom doesn't have the two-wire loom for the gauge, so this was removed and attached to the old loom.  Green-Black wire gets a straight through connector to the same colour wire from the engine bay loom (then onto the sender unit).  But there's no where to connect the remaining single black wire at the fusebox.  Like many cars, this has been tampered with and i can't find the wiring diagram for this particular late 1600GTI model.

Can anyone advise where the black wire goes, or provide a wiring diagram?  Is it just an ignition live? (It's a replacement fusebox too: the last fusebox had a relay that doubled up with the black wire: this wiring is now integrated into the new loom plug and the fusebox)

thanks