I looked into the custom fit setup for in dash. Unit mounts to firewall of 67-72 with existing holes. They give you 4 rectangle vents. If this is the one i am thinking of, you modify the oem non A/C heat defrost temp slider lever to add in a switch on the back side. So everythings in the dash using stock heater slider controls. You prob have to slide the lever to full cold to close the switch and make the compresor run, uses the stocker 3 speed fan switch in the dash, and of course the sliding lever that goes from floor air to defog now runs to the new under dash unit. I believe in the middle setting it now becomes dash air. I want one of these for my 67 cuda.