The AC drain line is under the center of the dash, about 6-8 inches behind the firewall. The likelyhood that water from there will find its way to the right rear seat is remote. Run the AC and see if water drips down from the car to the driveway on a humid day.

The sun roof has 4 drain holes, the front 2 are easily seen and drain down the A pillars. Take a 1/2 cup of water and pour it slowly down each hole to make sure the water pours onto the driveway immediately. If they are clogged, using a somewhat stiff wire, like 14 gauge, carefully try to ream out the hole. Usually, if blocked, the first 6 inches is where the block is.

If the two rear drains are messed up, you cannot access them from the top. You will need to remove the inside panel on the left rear to expose one of the drains. It is possible that the drain connection has come loose and is allowing water to come in.

For drying the rear carpet, you should think about lifting up the carpet and trying to dry out the sound insulation padding under the carpet. That will hold a ton of water for a long time.
--
Klausc, a Volvoholic

This has happened to me a few times. For my 2007 V50 it was the drains that channel water away under the moon/sun roof. I run a stiff wire probe (10 guage copper) down into the little drain holes to loosen up any smutz and then pour bleach water into the trough slowly and let it drain away through the holes. I have noticed that the problen could be worse when I park with the front higher than the back. It usually happens during a heavy rain. To get the water up I use a wet/dry shop vac. I put the front seats forward and there is a seam in the carpet where I can get to the water better, but I also vacuum water through the carpet itself. I have also had saturation in the front floor wells and extend to the back seat. Check your front floor too. There is a lot of insulation under the carpet that could hide the waters presence.
Good luck,
John