My 35 Dodge (from years ago) had small slots at the bottom of the windshield that were cut into the garnish moldings. Those slotted vents were supplied with fan forced warm air from a tube connected to the heat unit. I can tell you the warm air forced through those small slots cleared the windshield very quickly.

I suggest you use the area under the dash for heat/cool and windshield wipers, and run the electrical not needed in the dash, to under or behind the rear seat. The voltage in the wiring won't care how far it travels before it reaches its destination. Warm air from a heating unit can cool pretty quickly in a cold car, you want to keep those heat caring hoses as short as possible. Gene