If you have any water inside the passenger cabin, or in the trunk in the case of the sedan, you will have windshield fogging problems. The sedans like to develop leaky trunk seals and collect water under the trunk mat and in the bottom of the spare tire well if the drain hole is blocked. The wagons can collect water behind the kickpanels in the cargo area and around the tailgate area. Also check inside your spare tire compartment.

I live in the Pacific Northwest and drive my sedan and wagon year-round.

I set the leftmost most vent control all the way up and the center control in the middle or all the way down. I keep the heat turned off most of the time. Basically you want to set the two vent controls to give the maximum airflow out of the defroster vents.

I also crack the driver's side vent window open; this is essential for keeping the side window clear and also helps to defog the windshield; use of the vent windows is also what Volvo recommends in the owner's manual for defogging the windows.

Remove the fan from the heater box and clean all the debris off the top of your heater core. Remove the cowl vent and clean all the debris from under it as well. Make sure the two drain hoses are intact.

Replacing old, dried out and crumbly seals on the flapper doors of the heater box will improve the performance (airflow) of the ventilation system

Driving in a torrential downpour will tax the ventilation system; I would keep a terrycloth rag or washcloth in the car for such occasions.

A small electric fan on the package shelf will keep the back window defogged.

Bill