Thanks everybody I'll try everything Bepa

 

My advise is to forget all the fancy stuff... after living in the great white north all my life there is only 1 thing to use:
Get an empty spritzer bottle and fill it with Winter Grade Windshield Washer Fluid spray as Windex inside or out and wipe off with a microfiber. It can be used year round and you can leave it in the car. It even helps when the windows are frosted over on the outside... just spray and it does the hard work for you.

 

Not knowing what state your from.. Makes it a minor mystery..

Since this is obviously a used car...
It could be a couple easy things to remedy...

My first guess... Is that theres possibly some cold air leaking into the cab...thus adding to the humidity inside the cab of your Monte...
Thus causing some of the condensation...

2nd:
It could also be that at one time it could have had a bad heater core and the end result is that someone didn't clean them properly after the repair...
As long as there is no after market tint on the glass... Windex with ammonia and some newspaper to wipe them dry... Should end the problem and the glass should remain clear from there..

3rd:
If you smoke... and you smoke in the car .. that also adds to the problem...

Hope this helps!

 

I see that Space has already mentioned it but really turning on your AC with the heat up will dry the air in your car and help prevent fogging

 

Make sure your a/c isn't on max, or recirculate, if so the air that is in the car, stays in the car, it will actually increase the humidity. You need to set your climate control to allow outside air in. You car should automatically do this when you switch to defrost. But if your car has the option for recirculate or fresh air in, make sure you are on fresh air in.