Hi, I have just fitted a tow bar and wired in the electrics to an o4 Focus C-max. I had wired up trailer socket to a 94 Escort and 98 Scenic before, but nothing from this century.
I used a six-way relay which I bought from a local trailer place. The 6 input wires to the relay are spliced to the car's existing lights (right indicator, left indicator, brake, left tail, right tail, fog) , and the wire from the trailer socket is wired into the relay output. Connect the two earth wires to the car metalwork, and 12 volt + supply comes from an existing red wire you will find behind the nearside luggage compartment side panel. The relay has a build in buzzer that operates when the trailer indicators flash.
Towsure part number EL123 Combination relay but no doubt similar 6-way relay are also available from caravan/trailer shops.
Hope this helps. Cheers