I know little about sunroofs on this model. Generally though I have seen many that are very slow or that will not complete closure without a hand.
I think you could remove the motor. Check that it is not shorted by operating it plus lubricate it as well. Get the cable out of the housing and figure out how to get some solvent soaked device through it to clean the old grease out. Simular treatment for the rails and guides. Clean the cable itself as well. Lubricate everything with the proper special lubricant or something simular if you can identify the special qualities. Should work when everything goes back together in most cases I suspect. The old grease just becomes stickey or hard I imagine. Others will post actual information from experience. Two of mine are waiting for this treatment but still semi usable.