the reverse switch is on the top of the gearbox,
if you look under the air filter you will see it, there is a grey push connecter in it with two wires leading from it, the connector isnt the easiest thing to get of, i used a pair of plyers on the top part of the connector, dont pull the wires. you will find it easier to take the air filter housing out, just undo the large jubilee clip at the back of it, disconnect the wire to to the sensor and it should pull out.
the new reverse switch cost me £7.90 ish GSF (german swedish french) parts.
it took me approximately 5 mins to change
Andy G, February 2011