altagest:
Don't think it's a matter of contrast, since i grounded the contrast pin too and gave him directly 3.3V, nor with the trimmer.
Another tip: do i have to put a resistance on the backlight led or it is already "slowed down"?
First, provide a picture how you connected the trimmer. Neither grounding the contrast or giving 3.3V was correct.
Second, yes you need 300 ohm to 500 ohm resistor to "slow down" the back light current.