You've been very helpful, even if you're not right here.
I've been doing a couple of test and I think you're right. I unplugged the 3 wire connector that went into the fan switch and tested it with the multimeter. I got 12 volts when testing green-white and green-blue. Therefore green is always energized and white and blue are the ones that ground the circuit. Then I used a piece of wire to short circuit and found that green-blue is low speed and green-white is high speed. Then I unplugged the relay wires and plugged the multimeter instead. When I short circuited green and blue (low speed) I got 12 volts and got no volts when short circuiting green and white (high speed). Also, the blue wire goes into the relay and comes out as the white wire that goes into the fuse box while the white (high) goes directly into the fuse box from where, I would assume, it goes into the fan.
So indeed, as you suggested, the relay is nothing more than a resistance that reduces voltage to the fan when going into low speed.
But then I did this test. I short circuited green-white to get maximum fan speed (which I got). Then I started turning on every electrical device inside the car (Radio, heater, lights, braking lights, warning lights, etc.). As I started turning everything on I could hear the fan reducing its speed. Finally, when everything was on I tried to turn on the car and, as I suspected, nothing happened. Once again I couldn't even hear the starter moving (although everything inside the car was shut down as I turned the key, which is what any car does when it starts). tried again and it finally started.
Which brings me into this little detail. Since I bought the car over 5 years ago (I bought it second hand) I always noticed that the windshield wipers were somewhat "weak". They would always have a little "shake" when operating. Recently I had to replace the alternator's regulator and I noticed that as soon as it was replaced the wipers were opperating very strongly as they never did. But after a while they went back to their usual operation. I don't know if there's perhaps some kind of "power management device" that is failing and isn't sending enough power to certain devices.