also if you have a sunroof check to see if the rain spouts are clear of dirt build up, and to see if the tubes are headed down out the car and not draining inside . most sunroofs have 4 drains spouts,( how can rain get in with the sunroof closed) it does, the seal hold enough water out but you would always have water that leaks in.if you dont have a sunroof SCOUTSOUT AND CHINSAN are in the right direction.
you can also go into a automatice car wash and hop into the back and check wheres its coming from.