Just the same i've found in my GTV 2000/1. The thicker black wire is also on the battery side and is connected to both contacts of the rear demister.
Is it possible to cut the thicker black wire on the battery side and use it instead of the braun/white wire?
Check this with a meter set to continuity (OHMS).. or buzzer..Also not going to mention colour codes, were talking Italian electricians here
You will need to identify the wires that are connected to the heater element tabs inside the rear screen.. Usually THICK and black..
You may need to disconnect both wires at the element tabs to do this. Reason being that the element is such a low resistance they will both 'buzz out' as the same wire. (that's what's confusing Redzepp)
Carefull don't break the tabs off!!
If you can't find the THICK black wire on the battery side, get your fingers up inside the hole and feel for it.. you may have to rip the back seat out to find it. (Seanhump).. sorry mate!
OK HERE GOES..
There should be two THICK wires connected to the heater element tabs.. one each side.
One will go to earth and the other will be the 12v feed wire from the fusebox.. You have to be absolutley sure which is which.
You will need to cut into the 12v THICK feed wire to provide a supply to one side of the relay coil. (Check you have the correct wire.. Voltmeter)
Using your own THICK wire connect the other side of the relay coil to earth. The relay should then energise (click on) when you hit the demister button.
Using your own THICK wire run a feed from the battery Pos terminal to one of the relay contacts..
Also using your own THICK wire run a feed from the other relay contact to the THICK heater element wire that you have previously cut into.
Fit a fuse carrier and fuse.. Check the integrity of the connections especially the THICK wires.. (give them a good tug)
DISCLAIMER..If you haven't a clue what I'm talking about go find a sparky/electrician and show him this.