Heat stops working in idle but works at speed

When van is idling it will only blow cold air most of the time sometimes we do get warm air. But you can be driving down the road with hot air coming out vents and come to a stop light and it starts blowing cold air. I have asked this question to several sites with all giving the same answer that it's the blower motor not working and the hot air at speed is the air being blown into van while moving except for I have had blower motor out and turned it out with it in my hands and it works fine and changes speeds when you change it on the heater. A guy at work today said he had this same problem in an impala and the shop said it was a cracked cross over tube. Is this possible cause on my van? If so when is it located at and how do I change it? I have also made sure that the coolant is full and I see no leaks. Van also does not overheat or anything else like that any idea other then a bad blower motor would be helpful thank you