Ah, one for me I think!
Sebastian,
Hi, and welcome to the Fiat Forum.
The problem you describe sounds like the 'reverse inhibitor cable' - basically there's a cable which is supposed to get pulled when you push the gearstick down, and if it doesnt, you won't get reverse.
Take the gaiter off the gearstick (it just pulls up) and you will see the cable running up the left hand side of the stick. Push down on the gearknob and you should see the cable operate. The other end of it is on top of the gearbox, held in by a small (10mm) bolt. If you take it out you can check if the cable is snapped or what.
There's a couple of ways to fix it, the proper one being to buy a new cable, or the fiddle - on the gearbox end pull the plunger bit up and secure it (a piece of electrical terminal block should do) and bolt it back in.
If you go for the fiddle, do note that it will be possible to grind against reverse when pulling back the stick out of 5th gear, so it should only really be used as a temporary fix.
Hope this is of some use to you.
Pete.
Cinq sporting 1242cc
http://mysite.freeserve.com/close_to_cracking_up/