I have checked the resistance of the heating wires in the seat shows zero ohms, the voltage on the other site of the connection is 11,5v. What can it be?
I have a 20 years old Fiat Punto 60 (1999)

I would like to find the fuse number and location for this problem. FIAT Punto 60 (1999)

I like this car a lot, it has been like family member since 17years (there was an owner for 3years before me). I have a Citroen of the year but the Fiat Punto 60 is my favorite.

If you are getting 0 ohms on the seat side of the plug the I suspect that the heating element has failed and is shorting. I'm sure you should get some resistance however low.
The 11.5 volts may also be low however depending on battery voltage. Does this switch to 0 when you turn off then back up when on?