I have this mirror in two of my chevy trucks. The ambient temp sensor is what gives you the temperature reading in the mirror so if you have already replaced the sensor, I would say that the sensor is getting some additional heat from somewhere. I wouldn't put it anywhere near the radiator. I mounted the sensors to the middle of the grill in both of my trucks and my temps are pretty much right on.
2014 GT, Automatic, Sterling Gray metallic, GT500 AB. Ford STB and aluminum diff cover. Bob's oil catch can.