1995 Ford Escort excessive condensation inside

My Ford Escort wagon defogging system seems to be working fine, fan works, heat works. But, I've had, in the last month, an excessive amount of condensation on the inside of the windshield, and all the other windows. When I run the defrost system it clears the lower half of the windshield but doesn't clear the whole thing. I orginally thought the problem was with the defogging system but I believe it is working fine. The problem must be that there is an air leak along one of the seals of the windshield or rear window, and water vapor is getting in. However, there is no obvious water leak, and no obvious wet spots along the window seals.

How can I figure out where the moisture is coming from when there is no obvious leak and the windows keep fogging up?
Any thoughts would be appreciated.
JW