If you tap into the defrost ducts to feed other vent openings you would likely reduce the effectiveness of an already ineffective defrost system. It's not exactly a powerhouse already.
But you would not necessarily want air blowing out at the passengers when the defrost is on anyway unless you think that additional flow direction will help the defrost function?. Probably better to open up some new ports in the side of the plenum and run your new vent hoses from there.
If you just want heat to come out of the floor register and the defrost ports at the same time, you simply pull the Defrost knob out half way. Or any variation thereof...
If you're thinking of cooler air in the summer, you have your floor vent on the driver's side of course, but you can also use the heater system for a little extra at your feet as well. It's not super powerful, but if you just leave the TEMP knob pushed all the way in to cold, but pull the heater control knob (the one that says "pull for heat") all the way out, your passenger side inlet for the heater system acts like a fresh air vent. So you would get a general flow in the foot wells, and then the larger venting flow from the driver's vent door, and then some more perhaps using the wing vent windows.
But as a defrost option, I'd leave it as is and use the heater flow instead. And if you have not already then use one of the heater blower motor upgrades (like this:
https://www.wildhorses4x4.com/product/Heat_Blaster_Kit/Bronco_Heater_Parts and others) to get more flow volume.
I figure anything you can do to make the defrost ducting more efficient is a good thing. I'd like to see a diffuser focus the air flow in a more broad pattern so you don't get two small circles of clear windshield and then wait 10 minutes for the rest to clear up!
Paul
