[AnotherWagonFan]

Pull the A-pillar trim. It's quick and easy. 99% that the drain tube has separated from the rubber exit point to the outside. It's a quick fix to push it back in. There are a ton of different "fixes" for it to ensure that it doesn't happen again. YMMV...

Once that's done, pull your interior...you'll need at least one day, maybe more, for it to completely dry. The front seats are an easy removal (4 main bolts, the air-bag and seat-electrics connection underneath, and then release the seatbelt from the side).

Then, start removing the carpet pieces. It's not that hard and there's only a few connectors, and the door-sill trim pieces. The rest is just form-fit and slides under the interior trim pieces.

You will not believe how much water can collect back there until you see the amount of open-cell foam insulation that is down there. I literally pulled 1.5 gallons out of my floor pan, and squeezed even more out of the foam while helping it to dry in the sun...

If you're in a humid climate (or it's just cold and wintry, you may need to leave the carpet out for a while. You can reinstall the seats while it dries, so that you can drive the car. Just expect it to be louder and more resonant while you drive.

Lastly, on the right-hand side of the center console, you'll have uncovered a plastic drain tube (two white ends and a black, accordion-style connector pipe. That's the A/C drain tube from the condenser. Pull the black tube off and clean it and the white elbows out. They are most likely filled with decaying pieces of grass, leaves, pine needles, etc. That can cause as much flooding as a disconnected sunroom drain, even when it's not raining.

Once the interior is dry, the under-carpet foam is dry, your sunroof drain tubes are connected, and the A/C drain is cleared and reassembled, put it all back together again and enjoy...

Some of the water left, after I had already cleared out a bucketful and then remembered to take a picture:

The A/C drain that should be cleaned... Volvo part # 30630756:

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A quick note to add to this as I've read in many other threads. Make sure to disconnect the battery before pulling the seats out and do not reconnect that battery until the seats are reinstalled and connected to the wiring harness. Otherwise you'll end up with an SRS warning light that will need a dealer tool to reset because of the airbags in the seats. Just got done with this same process on my V50.