Mystery wire in Horn loom, anyone ?

Hi fellow Mk1 owners,
I'm making some progress with fixing the wiring in my pride and joy but can't work out what the (red/yellow) wire shown in the pic(s) is for, does anyone know please ? Black/yellow is horn positive and brown is earth I believe.
I've had a look through the Haynes wiring diagrams but am still none the wiser and would be very grateful to find out so I can move on to fixing the wipers ..

Thanks in advance,

Where is this cable bunch located?

Year and type would be nice to know as well   As IIRC the Black/yellow is full time 12V+ to the horn in the engine bay, and I bet the Brown is switched ground from the horn relay.

Thanks for your replies.
The wires emerge from a black plastic sleeve next to the (dual) horn ( i.e. left hand side of the radiator) area and it is on an '83 LHD Mk1 Golf GTI.
The red / yellow wire is the mystery, and yes the other two are for the horn, which is working.
Just came across it as I was re-fitting the bumper and horn after removal for some cleaning.
Thanks again for any ideas.

red/yellow maybe power to the Brake light switch?

From the wire colours and the location on the car, the yellow/red wire would be for the coolant temperature sender, I assume the one on the radiator.

Thanks for the replies, I will try and trace the wire back to the fuse box based on your suggestion of the radiator fan switch which the Haynes says goes to C7 or the coolant sender which apparently goes to D29 and update this thread.