Thanks for your explanations. It looks like the resistor was broken. I've changed it and I've also checked why my a/c didn't start and now it is. Now, when I start the a/c the fans start in low speed. It was only cold outside in the last days, so I couldn't check that the fans start when the engine gets warmer, but I think it's ok.
The strange thing is that the fans start after I turn on the engine, for a couple of minutes. When it's cold. Is this normal?
Another interesting thing I've discovered... I don't have the fan's 30A fuse. Not that it's missing, but there are no connectors for it in it's place.