2000 Dodge Durango Whats wrong?

I bought a used durango back in feb, 2006 and it had about 52,000 miles on it.
about six months later it started to over heat when I checked the coolent it was low so I topped it off and no more problem since I lived in Florida at the time I never had to use the heater we only used the AC then we noticed that we could smell the coolent in the cab while the AC was on but no over heating until it was low on coolent as long as I kept it topped off it did not over heat then my water pump went bad I replaced the water pump and the heater hose and the upper hose as well as the thermostat no problems last month in Sept 2008 we moved to PA and when I tried to use the heater it comes out warm not hot and the windshield is always fogged up it does not over heat I had a fluid exchange done at sears about two weeks ago and I checked it it was about a 1/2 gallon low.
now we see some type of vapor comming from the vents when the air is off or not in operation this vapor is oily and gets on the windshield.

what is wrong with my truck please help