I take it you have rain drops on the seat, I once had this, I removed the internal panels and got the wife to pour water on the roof and saw where it came in and filled the hole with black mastic,if you open the roof and stop it half way you can check for any splits in the rubber,a new seal from lexus was about 167 if I remember right, but it was a 3 day wait, and it took me a morning to do the change over,it was fairly tricky to do though, if you opt for a new one, fit it in place and do the bolts before removing the tape cover.