Just my $.02 worth on the subject, but I think the point of mounting the sensor on the hump and only attaching the clip and screw to the "dash mat" is to allow the sensor to most accurately sense inside cab temps. The dash mat keeps the bulb from getting influenced by exterior air temps from the metal firewall panel. Same thought on the mounting clip and screw. The screw not penetrating to the metal firewall panel, keeps it from being a path to transmit the temp of the firewall. That heat/cold transfer would confuse the bulb into think the interior temp was either too hot or too cold and effect the function of the heater control.
Make sense???
