Why heater only works when car is in motion?

What is your question?

Auto Heater takes significant time to warm up, and then works only while car is in motion. When we stop at a light, or sit anywhere in idle, heater quickly changes and blows only cold air.

What makes this problem better or worse?

Idle, every stop light.

How long has your 2013 Nissan Xterra had this problem?

Since we bought the car in 2015