Anyway, on the 1998 Frontier the vent flaps are controlled by cables, and mechanism on the driver side of the air control box in middle of the dash, see diagram/photo.
I took at look at this today; first thing I did was to spray some cleaner/lube over the mechanism. Then I had to remove several dash and trim parts. The Haynes manual I have was actually considerably better for trim parts removal than the official factory service manual I have downloaded. Anyway, the mount for the cable sheath had broken loose, because the mechanism had gotten dirty and gummed up over the past 27 years. I cannot fathom anyone would go under the dash to try to clean and lube that every few years. Anyway, after getting enough access, I somehow freed up the mechanism and then used trial and error to get the air coming out the main vents only to driver and passenger, figured we didn't need under-dash or windshield air flow, as long as the AC worked, and heated air come come out the same vents. This took 2 hours, but was under the carport.
So after buttoning that up, I replaced the engine throttle position sensor with a new one, the correct part. And then had to turn off the air bag flashing light (I'd unplugged the harness passenger on-off connector to remove that trim piece). This part took another hour. It was only 109F in Phx today, now back up north now.