Moon roof, heck yeah use that. It will work better than a side window, but it only needs to be cracked a little, not full open.
I would think a dehumidifier and leaving the windows in the car open while parked would be good. You can open rear windows in the car to minimize discomfort while driving. Perhaps ground moisture is coming up through the garage floor is also a source of moisture, maybe a sealant or epoxy might help. Your recap sounds good.
Yes, if parked in the garage with the windows (vehicle) open humidity will seek out the coldest spot and move there. If there are windows in the garage they will draw the humidity to them unless they are multi-pane. Everyone my age will remember old refrigerators that accumulated all the frost on the freezer. If an ice cube were dropped on a shelf it would vaporize and move to the freezer without ever becoming a liquid.
I lived in a cold climate in Colorado and never had this problem in any of my vehicles. Colorado is a low humidity place tho, so maybe that’s different than where you live OP. When leaving work sometimes the windows would be completely frosted over, so I’d have to start the engine and run the heater and defroster on max, turn on the rear window defroster (electrical) while I scraped the snow & ice from the outside of the windows, but by the time I got back in the car and ready to go the interior of all the windows would be pretty much completely free of frost & fog. hmmm … so what’s going on w/your car? …
The water needed to create the interior fogging has to be coming from somewhere. Ask your mechanic to make sure you AC drain isn’t plugged. I realize you aren’t turning on the AC, but when you use the defrost mode, it turns on the AC without you knowing about it. Peel back the carpets best you can to see if the under-padding is damp. And check the trunk, especially the spare tire well, to make sure it is dry. If everything seems ok, as you approach your office in the morning roll down the window and air the inside of the car out for a few minutes maybe, see if that helps.
About heating the garage. If you live in a place where they use salt on the roads, a heated garage can be problematic, since the heat combined with the wet road salt sticking to the underside of the car causes the rust to spread faster. Suggest to turn down the heat inside the garage to no more than 50 degrees, 40 even better.
One more idea, be sure to use good high quality water impervious floor mats, the kind that don’t allow moisture to move off the mat. Mine has 2 x 2 inch squares that hold any water that gets on it in place in those wells .
If the garage is newly constructed there is likely a lot of humidity coming off the concrete floor and all the new building materials. In a few years it will be a lot drier, but right now it’s going to be very damp in there. I’ll bet if you left the door open for a little while there would be ice forming on the floor and walls.
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Good point. One wayt to test that idea, OP could park the vehicle overnight outdoors for a week or two, and see if the window frost problem goes away.
I used to crack a window on the sunny side of the car and the moisture would blow out. If you do that on the shady side you will frost that side. I could see a cloud escape if I opened the trunk in January so I would sometimes leave the trunk open for a half hour in front of my house. I had no troubles. When I sold the car to a guy who had to park in the shade, the rear window was always fogged up. Sometimes you will see condensation on the inside of the trunk lid. If you own a Toyota you might never find a solution.
Yeah I have that problem too if I don’t drive the car long enough to fully warm the interior up and dry the moisture. So maybe going to have to let the car warm up in the driveway for a while. The other thing is you could have a little higher moisture level from the new garage-wood drying out, concrete drying, and so on. That might improve a little but you’re still going to need to drive the car farther I think.
It does. I’ve parked the car outside for the last three winters, plus the first half of this one, and never had frost on the inside except for a couple of occasions when I had a lot of people in the car and it wasn’t warmed up yet. The problem only occurs when parking in the warm, relatively high humidity garage.
Makes sense. My drive to work is only about 15 minutes, and that is the farthest I ever drive. The grocery store and the like is only about 5-7 minutes away, so not a lot of time to make a difference. We’ll see what happens with the other suggestions.
I guess if you can shut off the heat to the garage on dry, clear days you should do that and open up the door so it can air out. When I lived in New England I used to open all the doors and the trunk of my cars on cold clear days.
Not really an option at -20º - -40º F 
You’ve done the proper experiment OP to prove the problem is caused by parking in your new heated garage. I’m thinking the cement slab is still outgassing some water vapor as it cures. That should subside w/time. I’m not sure if reducing the temperature of the garage would work or not. Maybe you’d have to increase it, not sure. Didn’t get good grades in chemistry when they talked about the ideal gas law … lol … I guess you could try it both ways, see if you notice any difference.
Beyond that, seems the only solution is some combinati9on of airing out the garage and airing out the car. Both difficult to do in those temperatures. You may just hive to live w/it this winter, and hopefully the cement will be totally cured by next winter.
hmmm … here’s something … google how to prevent fogging on glass. When I go skiing, to prevent my sky goggles from fogging up and not being able to see where I’m going, I wipe them on the inside with a special cloth that contains anti-fog chemical. I buy that cloth at the ski shop. Works pretty good. I’ve also just wiped them with a weak solution of water and ivory soap, and let it dry. That works pretty good to prevent fogging too, but it tends to not be quite as visibly clear as using that anti-fog cloth. Worth a try maybe.
Parking outside results in using the cars heater more. Using the heater helps to dry the floor inside the car and flushes out respiratory moisture. Whether you park inside or out use the vehicles ventilation system to keep the air inside the car fresh.
Reminds me. Remember they used to have those oval shaped plastic window things that would stick on the inside of the window? They would prevent the frost on the window so you could see. Used to see them mainly on older cars, trucks, and jeeps. No idea what they are called but I’ll bet they are in my old JC Whitney catalog.
So yesterday night I left the windows open while parked in the garage. I didn’t close them until I was outside and the interior temperature of the car started feeling cool (that didn’t take long), at which point I closed the windows and opened the moon roof, which I left open for the duration of the drive. Heater on a comfortable temperature (full hot would have been unbearable). Just before I got to my office, I cracked the windows again.
All that may have been overkill, but it did achieve the goal - when I came out from work, there was practically no frost on the inside of the car windows. I think I’ll still look into getting a dehumidifier once the garage area is closed off from the rest of the house - hopefully that would prevent the need for such drastic steps 
I wonder if your garage is tied to house thermostat. If the garage is a separate zone maybe a lower setting might help.
If the interior moisture recurs . . . ( I did this for our 06 Escape hybrid. )
Put some white school chalk in various places inside , like the door pockets and seat pouches.
It is on a separate zone, and it might help. At the moment, however, we are working in there, so we want it warmer. Once the work is done, I’ll try dropping the temp.
Interesting. I’ll have to try that
Try setting it lower when you are not working in there.