Well I finally got around to doing it and changed the heater core. I took apart the old one and found the problem. The tubes were plugged. I wasn't surprised when I seen the design of the new one. The Tubes are very small. That was a poor design on Nissan's part. They should have never made the heater core tubes that small. There should be something upstream of the core that would protect it from plugging. But doesn't really matter all that I know is that I finally have HEAT. Thanks to all those that helped.