cme,
the switch is on the back of the gearbox rather than under the engine. It is easier to see with the n/s wheel and wheelarch liner removed as it's over that side.
With wheel removed look towards the engine/gearbox above the brake disc, you will see an electrical connector sticking out of the gearbox from left to right, with what looks like a bolt head underneath it. This is the reversing switch.
From the picture above that Decks posted, the shiny bit is inside the gearbox so you won't see that but the other end (white plastic) is the where the electrical connector will be attached.
Disconnect, unscrew, replace with new switch, reconnect........ job done.
You'll probably see the end is worn on the old one, mine had a little flat spot, LOL
Hope this helps and hasn't taught you to suck eggs too much,

