I have just changed the reversing switch on my 2004 Mondeo Est and it was much easier than I thought. You will find the switch on top of the gearbox which can be seen from under the bonnett to the right. It is under the air filter hose and behind the battery.
If you have small hands you will be able to get them through the gaps to reach the switch, otherwise you may have to remove the air filter hose. Unclip the wires from the top of the switch then use an ajustable spanner as I did, and remove the switch. You can try cleaning but the chances are that you will have to replace it. (The part number from Ford is 1441361 and cost £8.25 + vat).
Put the car in nutral and insert the new switch, don't over tighten. Reconnect the wires to the top of the switch. Turn on and select reverse, the reversing lights should come on.
It's as easy as that and should take you no longer than 15mins even if you haven't done it before.
Gerry
November 2009