Peter,Quite easy on the Focus, have a look up under the back of the glove box, towards the centre console. Its approx 1 inch wide by 2 inches long and will have a multi-plug with 4 wires and is screwed in with either a philips screw or a torx screw.
When you get the old one out, check the wiring plug for burns, its quite common for this to happen and Ford issue a modified connector and 6 inches of wire to repair.
Good luck
Paul