Most every vehicle I have ever worked on uses vacuum to control the air vents. As a failsafe if vacuum is lost then the vents default to defrost so you can see to drive. As a rule of thumb if only the defrost works then you should look for a vacuum problem. Since diesels don’t create vacuum they have a vacuum pump. You don’t have any info about the truck but since the post is in the 99-03 section then I assume it is a 99-03 which use electric vacuum pumps. May be a blown fuse or a bad vacuum leak or a bad vacuum pump. You should be able to hear the pump if you turn the key on but don’t start the engine and move the vent controls around. Mine is mounted near the passenger fender.