Rear Heater blows cold air.It blows hot air once in a blue moon.

So I noticed that as well with my truck. So If you put the back heater all the way to 80 temp and on feet only while your truck is warming up. it will start the warm up processes in the back. Once your truck is warmed up, you then can turn the switch over to heat the head (Not Feet and Head) just Head. and you will notice it to blow hot air. you then can adjust the heat temp and it will continue to blow hot air. Hope this works for you and hope it makes sense for you as well. Good Luck!