Installing a heating system behind your mirror is the most reliable way to avoid a foggy mirror. Typically, mirrors fog up because they are much cooler than the steam in a bathroom after a shower or bath. The cool surface of the mirror causes the moisture in the steam to condensate in the form of fog. A heated mirror will prevent this condensation by being as warm, or warmer, than the steam.
The defogging is concentrated in the area of the heating element, but may spread out as the mirror warms up over time. There are too many variable atmospheric conditions that make it impossible to quote a fixed distance of heat spread away from the element. That is why the defogger must be placed in the area of the mirror that requires defogging.
The electrical junction box should be installed inline with the Mirror Defogger’s lead wires and box. The box at the back of the defogger is meant to be located over the electrical box in the wall. This way it will be in the electrical box and not interfere with the defogger being flush with the mirror and the wall. The location where the wires leave must be entirely within the area of the new electrical junction box.