Side windows "fogging"

I have a 97 Venture. I love this van, but the only really annoying thing I have is that the side windows (primarily the rear ones) fog up when it's cold out. I have cranked the heat up to the point that I felt I was in a sauna, I tried cracking the windows open a bit but no luck. I am going to try some "anti-fogging" spray, but I seem to rememer that it doesn't last long and has to be reapplied very frequently.
If anyone has a suggestion on how I can alleviate this problem, I'd appreciate it. Is this a common problem with Venture's, or is it just mine?

Your rear heating blower fan is broke. Check your rear vents. If no air is comming out then the fan needs to be replaced. This forum has instructions to replace it just do a little searching.

that's one possibility, mine fogs up to, but it's just the front two windows. One other possibility is a minute pinhole in the heater core letting enough coolant into the air to fog the windows up.