My thought was to have more visibility for other drivers than just the stop lights. I have looked at how other vehicles have the third light mounted, I was more concerned about a minimum height requirement.
Looking at other trucks and how there third lights were mounted I decided that mounting the light below the window at the belt line would not be too obscure. I want something that doesn't take away from the clean lines on the body. Mounting the light flush with back of the cab would keep it looking clean.
I have original style LED tail light for the rear.