Near side rear arch wirring loom.

[Boatbuoy]

Still trying to put my car back together after the 'body off' refurb.

In the near side rear wheel arch is a hole through which the tank to pump fuel line passes. A number of electrical connections also come through, namely the wirring for the fuel pump and speed sensor. But I have a 3rd spur on this part of the loom which has a waterproof plug (like the speed sensor) on the end. From memory one of the wires is yellow. Any suggestions as to where it goes, or what it's for?

Best Regards,
Olly