Urgent Help Please - Red/Yellow Wire

Hi Everyone,
I have just got "Bluey" (1990 Dual Cab Syncro) going after many
months of "resting". I removed the engine to try to stop overheating
after loss of coolant. I persuaded myself it was head gasket however
I didn't really find too much. Anyway, the engine is back in and

However, I have a spare red/yellow wire that I did not tag and seem
to have inadvertanly pulled off (no terminal)when I removed the
engine. It appears to be associated with the cooling/temperature
gauge. The gauge does not work and it is not running quite right. It
is a single wire with black covering emerging from the harness on the
upper, left, front of the engine bay. The temperature wires I know
are a sensor and temperature gauge sensor *green) both located on the
thermostat housing at the front left.

I'd really appreciate anyone who knows or could look at their car
and let me know where it goes.

I am really excited about driving bluey again and camping down south
at Nannup for the folk festival this week end if I can get it going.

Kind Regards