Hopefully it's just a coincidence that it started when you plugged the block heater in, otherwise it could mean moisture is getting into the cab via a leak somewhere, but that's a stretch.
Keeping your windows cracked will work. The key though is to get the air inside the car as dry as possible before you park it. Driving with your climate control on defrost will help and usually a vehicle will turn on the A/C compressor at the same time to get drier air. Also don't recirculate air, always draw it in from the outside.
Failing that, you can put a coating on the inside of the windows to prevent frost from forming. I haven't done that myself so I can't really give advice on that route.
__________________
|