Having had a look behind the trim panel recently on my car to investigate what's needed; here's what I found.
Sorry there's no pictures but it was dark and the street light isn't much use and I was struggling to hold the torch and manipulate everything else.
The C111 connector is the largest (widthways) one nearest the edge of the door pillar with the wires from the loom being vertical. There's a smaller (widthways) connector mounted at angle below and forward of the C111 connector.
Do not unplug the C111 connector unless you've already disconnected the battery; make sure you have your radio code beforehand !
The C111 connector on my car had a a light blue latching handle which swings upwards and outwards (i.e. away from the floor and pillar) which releases the lower moveable half of the C111 connector downwards. The upper static half of C111 is fixed to the door pillar with two scrivets (one either side of where the loom enters the connector body) and a large plastic dog-leg clip (hidden at the back) that's part of the upper half connector's body.
If you want to remove the C111 connector from the door pillar then :-
1) Unlatch and disconnect the lower half of C111
2) Undo and remove both scrivets from the upper half of C111
3) Wiggle the upper half of C111 upwards and slightly outwards by angling the top edge away from the door pillar
Since my 2008 Focus is a Titanium I was rather hoping that I'd only have to add a few of the wires to only one side of the C111 connector; no such luck though :-(. The connector and wires for the ESP switch are not present behind the switch panel either (I checked the two taped up connectors; they were both for the heated seat controls) so I'll have to pick up the switch illumination feed from one of the other switches. Also there are no wires or pins for ESP in either half of the C111 connector at all (1, 5 and 10) which means adding a three wire mini loom through the bulkhead grommet to the C830 connector of ABS module and the engine bay fuse box. See page 81 which is titled "BRAKES-ABS/TCS/ESP(MK60)" of the Focus Mk2/Mk2.5 wiring diagram in the pinned post by Preee.
As has been mentioned by others it seems that only an ST focus will have any of the ESP switch, C111, C840 wiring in place.
The Finis code for the ESP switch has changed and is only available with a silver finish; new code is 1559485 (for reference the old Finis code was 1492837).