Sundance

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.