First off, you need the wiring diagram, and the workshop manual, you should be able to find them here:
https://pchelper.org.uk/manuals.wotmeworry.org.uk/
It's a challenge for because I'm colour blind.
My 2010 GTS did not have a horncast light.
That wire with the white connector, if the wire is black and yellow, is the wire that goes to the rear tail light.
That's about as helpful as I can be.
Hopefully someone with better eyesight and more expertise will come along.
Good luck, fingers crossed.