1989 Cherokee 4.0...... I recently noticed that , even though I manually move the lever from vent to heat to defrost, it stays on defrost(all air going to windshield). The lever feels like it's engaging in each position but the venting doesn't change. Not a great explanation I know..... any help? ( if you understood ANY of this!!!) lol Thanks....

 

Find this line check for issues.

 

You are a vacuum guru.......I'll check it out when I have time tomorrow. Hope it's as easy as your last suggestion .....thanks again!!