93 Nissan 2wd pickup have defroster working but no heat through vents

SOURCE: @003 nissan sentra SER

a/c is active when in defrost mode to remove moisture from air. what controls the movement from vent to heat/ or defrost. a lever or switch.

SOURCE: 2000 gmc 2500 pickup heater fan works good but not a lot of air

http://www.fixya.com/cars/t1386554-91_ford Please read this post, I hope it will help you.

SOURCE: PROBLEM WHEN SELECTING THE DIRECTION FOR THE AIR

Sounds like you have a supply vacuum problem. When you lose supply vacuum to the control head it defaults to defrost.

SOURCE: ac only blows through defrost vents

I just found my problem that often brings in a Check Engine light, and reads out a PO 442 code. When my A/C is on my heater would start working because a vacuum operated control door would slam shut when accelerating. Also my cruise control would stop when going up a large incline.

the first thing to check, and easy is the cruise control for a bladder leak. Look on the top side passenger side of the firewall, and you will find a three way vacuum hose connection. One will be a very small black plastic line that runs to the cruise vacuum actuator. Pull this hose loose at the fire wall connection. Blow in this hose, and if it will not hold pressure, and in my case I could hear air passing through it is defective needing replacement. Block off the firewall connection for A temporary fix.

SOURCE: 1993 Nissan pickup, replacing starter

I have a '95. It's behind the passenger side tire. Remove tire and rock guard if you have one--couple of screws--and you can easily access it. two bolts facing rear of car, two connections up front. Honestly it was the easiest replacement I've ever done. Just disconnect your battery first.