Get a wet and dry vac, good fabric shampoo and do work. Best just to wet the fuck out of the seats, if there's a clear nozzle on the vac then you know when your done by the water colour you're sucking up.

If you're brave and have a dehumidifier then power wash them. I'm not sure how good it will work for alcantara but I've done a few car seats with a power wash and stuck a dehumidifier in the car for a few days and it shifted lots of shite.