New to this forum and have had many questions answered using this format for other vehicles I have worked on....Have 99 Cherokee Sport. All outlets work properly according to the control positons except for defrost which will work only at the low fan setting but in bad weather when you really need heat on the winshield you can turn the blower speed up and it closes the door to defrost. I have pulled the radio and control panel and glove box and can see the routing for the vacuum but it's pretty tight to do anything. I have searched these forums but nothing really addressed the issue that I could find. I can see the defroster door and it remains open at low fan speed and sometimes in second fan position but any fan speed above that seems to force the door shut. Any and all help greatly appreciated. Thanks.

 

I have the exact same issue on my 98 and am also looking for some help with this
Looks like a spring project to me

 

Been there done that. My issue was that the door would not open so I had no defrost at all. As you stated removing the radio really doesnt show you anything. So, determined that I was going to fix this, I proceeded to make my way to it. Well after removing the steering column, center console and the dash, I find that my problem is that the arm from the door to the actuator is broke off inside theheater housing. The actuator is actually working just fine. Well figuring I couldn't actually fix this part or replace it without removing the entire heater box (which I was not willing to do) I decided on some redneck fabrication. The blend door itself has a fairly large vertical tab on the top side of it. I just drilled a hole through it and inserted a piece of stuff wire. Then I ran the wire out the vent through the dash. Now when I want defrost I just push the wire down and open the door and pull it closed when I need heat on my feet.
The wire protruding from my defroster vent makes a nice conversation piece.