I took out the alternator and disassembled it. The brush holder was melted and the brushes were fixed in their place. I replaced it with a good one and mounted the alternator and tested it. The battery light is on again!
I took it out again and replaced the voltage regulator with an OEM new one. I tested the rectifier and it was bad, so I changed it with a good used one. I mounted the alternator and tested it again. Damn! The battery light comes on again!
I've tested the charging light circuit and it's OK. The grounds and battery mounts are OK. The belt is in a good condition and well tensioned.
So, what you think?