Thank you everyone, now it's "working" but I'm not sure why. I've started again from scratch with a fresh installation, and it's working fine out of the box, as @ChuckyP said. At least is working with the padtest iso, when I push on R3 it starts running the weak motor, and with L3 the strong one. I suppose that I messed up something while trying to install/uninstall packages and stuff.
However it's not working with Final Fantasy VIII. Vibration is enabled ingame, I've tried with several bios and two different copies from the game, without success. Anyway, if it's working with the padtest then it works, and based on other comments I suppose that this feature doesn't work for all the games, but I need to test it more.
Thanks again everyone for your help!