Sometime it feels that cold here in Minnesota :)

Your profile does not indicate country or postal code, so I have to assume that you are driving on the right side of the car.

If the light switch causes the dash lights to flicker, it could be in need of replacement. Verify that the dimmer switch works correctly by sliding it back and forth. It is best to park the dimmer switch at the 80% level to preserve the dash bulbs, especially if you always leave the headlights in the ON position.

The interior lights are supposed to have a delay when closing the door and dim slowly, after 20 seconds or so. If you lock the car, they turn off immediately. Easily checked, the latch on the door that strikes the B pillar has a small metal microswitch that can be pushed away from the body to simulate the door being closed. It is shown in the Haynes manual. In order to put the switch back in the open position, pull the door handle.

Wet floor... No, the drains under the windshield are nowhere near the air intake and relatively difficult to clog. 1 inch plactic pipes about 6 inces long and straight down.
But do check that the cowl cover is attached to the bottom of the windshield and covers the glass.
The foam underlayment - 4 P38 WWII trainers just flew over in tight formation! What a beautiful sight and noise!!
takes forever to dry out. If the Previous owner had a heater core problem, try lifting the carpet and placing newspaper down, thickly, and try blotting up some of the moisture. It took me 4 days, lots of newspaper, to get mine to damp instead of wet. Parking in the hot sun also helped.
To inspect the heater core, you will need a Torx screwdriver and then slide the center forward console cover out. Look for leaking antifreeze traces. Also check the level of the overflow bottle in the engine compartment.

Buying parts depends on where you live and shipping. I prefer OEM when getting critical parts.

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.