Neat car, but I am guessing it must be an underdash, dealer installed air. It is missing the A/C box on the firewall. That looks like a standard heater box to me. Cool car, all the same.
__________________
1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars
Here's the link. 
ontiac-Grand-Parisienne-with-factory-396-W0QQAdIdZ35694174
