I began having trouble defogging my 2003 Camry’s windows in December. The car interior is dry, door seals are fine, no leaks. I think maybe control that switches from outside air to recirculating air is bad. How can I test that without taking the dash out? What else could it be?
Is the problem defrosting or defogging? These are two different issues.
If the problem is fog, try running the air conditioner or cracking a window. The humidity comes from your breath. You can also clean the windows and use “anti-fog” wipes on the inside of the windows. If the problem is frost, you can use the rear defroster and an ice scraper for the rest of the windows.
Which setting are you using, outside or recirculated? You didn’t say. Outside is better for defogging.
“How can I test that without taking the dash out?”
I don’t know where the inside air intake is or where the door is, but they are usually under the middle of the dash or over the passenger footwell on the lateral side. (There is just too much other stuff on the driver side, like the steering column, brake and accel pedals,…) Often the dash has to come out, but you should be able to tell if it is opening by looking and listening.
Does the blower sound different when you change from outside to inside? Is it very cold where you live? If so, the temp of the air should change when you switch after the interior is nice an toasty. Stick you head under the dash and turn the recirc control and listen. You might be able to hear or see the door opening and closing. It is letting interior air in after all.