Fogging Problem

I have a 2002 Cavalier and for some reason my windows are fogging up on the inside of the car. My mechanic was unable to find any problem with my blower, heating rad and duct work.
When I am driving on the highway the back windows have to be left down a small amount to get the air to circulate and defog the windows. My front windshield takes a long period of time to defo also.
Does anyone have any idea what could be the problem?

so it is safe to assume that your fan speeds are working correctly and you are running it on defrost . your heater core is not leaking either. the only thing I would surmize is that there is too much moisture in the car from all the snow you get in Thorold. the carpets are likely saturated. when running in defrost mode the a/c should be cycling which would also help to get rid of some of the moisture