It is that time of year again and we are all needing our heat. My heat has not blown warm at idle since I last changed my coolant and never blown warm in the rear at idle or at speed. There are schools of thought that suggest the thermostat is bad and yours may be. I replaced my thermostat today hoping to solve the problem. Took 45 min and my new thermostat was in. I have to say that when I removed the old one and it was in the closed position I knew I had replaced it for in vain.
If your heat blows cold at idle and then warm if you hodl the engine at 2000rpm or more, then your thermostat is NOT bad. You just need to bleed the coolant system. See below for link. I really had to hold the revs up to get the coolant to circulate. Do it right and you will have heat in the front and the rear. Glad this is done now. Bring on the cold weather. I'm happy to answer questions if I can be of help.
http://www.justanswer.com/nissan/63m0k- ... -idle.html