Chuck,
I haven't noticed that effect in my 2004 Prius (I'll look for it), but when I was young and foolish I had a 1963 TR4 in Chicago. The rear window got foggy and I kept the top down all winter. With the toneau cover on and the heater fan on high, I could drive at 30
MPH in the rain or snow and not get wet. Stop lights were a problem though.
JeffD