How do I close the air vents in the doors of my '91 Nissan pickup?

What do you mean by door vents? I am unaware of any air vent in the door of a frontier?

We have 5 1/2"x3 1/2" louvered vents in the doors below the door handles. I'm sure they were factory installed because they match all the other trim. Cold air blows through them when we are driving. I've tried setting the air on interior circulation, but it doesn't help. Any ideas?

Interesting. My neighbors truck is void of these vents. I would suggest removing the door panel to see where the vent is being fed from. If you had to you could just block the vent from behind while that's off. Good luck!

Yeah, I had to deal with that on my 91 D21, those are part of the positive air circulation system, & it's annoying on very cold mornings. There's no shut off door for those, just gotta git creative in a non "cobb job" kinda way.