You can get the fiat manual on cd on ebay there are also printed workshop manuals on ebay for certain engines.
The reversing switch is at the back right of the gearbox just held by one bolt I think its a 10mm socket its sealed with an o ring wiggle it gently and lift it up. If the gear comes out with it don't panic its easy to put back in.
Test the switch with a multimeter first. The problem may be in the wiring or corrosion in the tail light contacts.
paddy, January 2010