Frost on the inside is VERY strange. You likely have a leak somewhere. Either the windows, doors, or sunroof are letting water into the cabin, or there may be a coolant leak in your heater core.
Do you smell anything inside your car? Sweet smell means coolant, musty smell means mold from an outside leak.
Typically there is not enough moisture inside the cabin to form frost on the inside–I have lived in cold climates all my life (PA, NY, MD) and have never seen frost inside any of my vehicles.
Leaving a window cracked may help but you should fix the leak if you in fact have one.