I have a silly question. There are three levers on the heater control: Heat, Temp and Def. I understand what the Temp and Def levers do but what purpose does the Heat lever serve? I see that it operates a trap door in the passenger footwell but I don't understand what it does beyond that.

 

IM sorry I dont have an answer to your question. I was wondering if yuo have any good detailed pictures of your underdash wiring/heater stuff. Im trying o figure my 67 out with nothing in it!

 

Look at an owners manual. It will explain the operation of the controls.

 

The "heat" control is what selects "off" and "on" airflow through the heater. There is no advantage to an intermediate position, it should be all the way up or down.