Good Question!
After a lot of thought, we decided NOT to open a connection to the dash top vent. Here are the reasons:
1. On the original GT this was an outside air vent, through the front center opening on the clip.
2. Because of the size of the vent, any air entering would come out a relatively low velocity, making it ineffective as a defroster
3. The pressure of the air coming out of the side vents would be decreased.
4. When is the last time one drove the GT in freezing or rainy conditions where a defroster was necessary?
However, if we were to add a vent, here are some thoughts.
1. The back of the vent butts up directly to the HVAC plenum, so one could simply drill a hole through each and air would be diverted to the vent. Some foam tape between the HVAC plenum and the vent would effectively seal the connection
2. A tube, perhaps ¾” diameter, could be connected to this opening, curving toward the sides of the dash, and the end then plugged. A quarter inch slot about three inches long along the top of the tube would provide a narrow opening so the air exiting would be higher velocity, making it more effective as a defroster. A section of electrical conduit may work well. (See pictures attached)
3. Once the grill is put back in place, the tube would not be visible. (Assuming the tubes are painted a matching shade of black).
4. Air should exit only on the left and right sides based on the design of the grill.