Thats dew. Just like you find it on the grass, you find it wherever humid air is cooled down. When the air in the car dropped below freezing, any humidity in the air is going to turn to water vapour , and on those cold windows, freeze.
If the vehicle has the fault (caused by age, not driving distance… the mechanicals wear out with distance … ) , the fault that causes this isn’t the door seals, its the air conditioner/ heater system leaking or blocked, so that water is either leaking into the system, or unable to escape the system.
The fan flows air through the air conditioner (cooler) first, and then the heater second, the heater which has hot coolant water from the engine in it.
The air conditioner’s cooler is small, and very cold, and it makes for very cold air…
Now very cold air can contain no water vapor, so water collects on the cooler and drips out below the car… But if it can’t leak out, the water may be forced through to the heater.
The heater may be leaking, and this will then cause the air in the car to become humid.
Well this is all conjecture, I am leaning toward the problem being that its not a severe winter there and you have warm humid days.
The main solution is to ensure the air in the car is dry before stopping.
- the air if you are driving around in freezing cold air, eg at night open the vent ( Some vehicles show it as vent open, and some as “NO recycle”) and turn up the fan - you can have heater set to any temperature you like, its irrelevant to the absolute humidity in the car … This Can’t work if the air outside isn’t almos completely dry (freezing temperature…)
Heated air may feel drying when its warm, but it can still have a fair amount of water in it… this is the difference between relative humidity (how “drying” it is at the temperature), and absolute humidity (how much water there is in the air, irrespective of temperature… When air is freezing cold, it contains very little humidity, and it wont be dropping what it has as frost or dew in your car .
- Drive with the " Vent closed" or “Recyle” and air conditioner on.
You can heat, if your car allows AC and HEAT at the same time (some cars will turn off AC when you require temperature to go up too hot… perhaps you just need to stick to warm air rather than cooking hot air…) ; the air conditioner acts as a dehumidifier, and the water drops out below the car… And then the heater warms it up to be dry warm air…
Actually you can do step 2 with the vent open but I can’t know if it works well enough in your circumstance. (depends on the humidity of the air outside and the flow rate of water from the cooler…)