Rain leaks and ends on bottom of right side seat.

Its sounds like the roof drain is clogged. The sunroof needs to be opened, jacking the car up first biasing it to the front poor water in each front corner of the sunroof opening channel and see if water flows out and drains down by the front mud flaps. Then bias the car rear ward and see if water drains out the rear both sides as you poor water into the sunroof drain channel. Probably take one hour. It is best to look for water leaks when everything is dry. First is the door seal or door, damaged in anyway? I have seen the seat belt having damaged the door seal on many vehicles. Do the doors close flush with body? Ensure the door seal makes good and uniform contact with the door (particularly important if the vehicle has been involved in an accident. You may need to pull back carpets pull back the carpets and sit inside the car (you may need a flashlight) and get a friend with a garden hose to spray water around the door jams to determine if the water is entering the doorway. Then spray water around the windscreen moving around the circumference of the windscreen. Look around the windscreen area and look up from the floor mat area (as best you can) to determine if water is leaking in around the windscreen seal. Is there any rust around the windscreen area or body work? If you have a sun roof there is a drain tube at each corner of the roof to channel away water. A blocked drain may cause water to fall from the sunroof area Water can also come up through the floor pan as you drive through water on the road, if any rubber bung or “seam sealer” has been disturbed or is missing.

OK, I had (had being the operative word) the same problem with my 2002 RX 300, original owner). I didn't notice it until this year with the exceptional rainfall. I'm fairly sure the blockage has been in place for maybe years as frequently I'd go out to my work parking lot and/or driveway and there would be heavy moisture on the windows inside. This year there was standing water that I noticed on the rear passenger floor. My driveway is slanted towards the street and I usually pull in front-end first giving the car a rearward slant (bias). I filled a small-necked bottle, like a plastic water bottle, to be precise when pouring. I opened the hood and poured water into each bulkhead slot under the windshield first letting each one drain out before testing the next one. Everything drained out behind the front wheels OK. Nothing on the firewall inside under the dash. Next I opened the sun roof, poured water down the passenger side channel. Drained out just behind the rear wheel on that side. Good. Did the same for the driver side. Nothing. It did seem like the rear passenger carpet was wetter though. Backed the car into the driveway, tested the front sun roof drains in the same way. All OK. I first pushed a fairly stiff wire up the tube to see if I could dislodge any blockage. The wire went quite a way then stopped. I didn't want to damage anything so didn't force it and pulled the wire back out. Didn't work. Went down and bought a can of compressed air (like the ones to blow off your PC keyboard). Pulled the car into the driveway front-end first with the rearward slant. Stuck the tube up the drain pipe, wrapped paper towel around it to force the air up the tube, then pressed the button for about 10 seconds. I got wet as dirty water came out. Retested that particular sun roof channel and all is good, I hope. Time and a little more rain will tell. Hope this helps. (Note: I stopped by a auto body shop to ask how they would unplug a drain. I stopped listening after they estimated 3-4 hours at $50/hour ripping out the headliner, etc. etc. etc.)