The switch installation is as follows (provided you would be OK with a simple on/off switch, and not a next-ignition-on reset to an ON position...):
Any one of the leads going out from a wheelhouse-anchored position, out to the individual wheel "tone-ring" ABS wheel sensor can be interupted (i.e. any one of the four wheels) -- i.e. an on-off switch added-in, to either of the two wires will do the trick.
In other words, you cut one of the wires of the two-wire pair -- and attach a wire to each end of the cut. You bring these two wires in to the dashboard area, where you want to mount the on-off switch.. and then you wire it so that "ON" is when the connection between wire-ends is made, and "OFF" is where the connection is broken.
Note that I don't know if there is a voltage present on the wire, a normal voltage or a milivoltage.. so I do not know whether (when you break the contact, or MAKE the contact) any switching voltage spike would be sent anywhere (i.e. possibly to the ABS logic module). I would say that the electrical engineers who are lurking on this forum ought to weigh-in on this point, and indicate whether some sort of voltage-spike-quelling technology (adding-in a capacitance or ???? or filter or ????) would have merit.
Also, note that the whole car is wired CAN-BUS... so I don't know whether individual tone-rings / ABS sensors are smart sensors or whether they generate milivoltages, when the tone ring moves past the sensor...?
Finally, on FiestaFaction, there is a thread on how more than one person used this methodology to cancel TCS action, on their '11 Fiesta -- on a car which had either Series 1 or Series 2 Body Control Module (BCM) program. Note that on the '11 Fiesta - Series 3 BCM program added TCS-off to the human/machine interface arrangement... so you can turn it off using "message centre" or the Fiesta equivalent thereof. BCM 1 and BCM 2 Fiesta's cannot be updated, 'cause the TCS Off warning light is not in place on their dashboards (like it is, in the latest cars); accordingly, Ford will not do the update. Still, due to the slowness of the steps required to cycle thru the Message Centre, in my mind, a dashboard mounted switch has merit.