Yeah, since you hear it grinding its getting power/ground and the motor is working fine.
As it hasn't blown any fuses yet, that means its probably not physical resistance, so my assumption would be the little gear on the sunroof motor is worn out and it's just grinding on whatever it is trying to drive. Either that or the thing it drives is stripped, which is less likely.
on my 5th gen coupe my sunroof used to like to stick and it would just sit there and whine. Turns out I just had to give it a push. I sprayed some lubricant in the tracks and where it seals and I haven't had a problem since.