Alright so I know the defrost vent has been a problem many a times, but this one is a bit different that I can't find an answer to. So I had the defrost stuck on problem and I (think) I fixed it. It works fine and changes fine on the dash and all settings work EXCEPT on the highway... Whenever I'm accelerating or randomly around 70mph the dash vents change to the defrost by themselves and will change back when I let off the gas. All the vacuum lines are connected right and are tight and this still happens, and only on the highway at 65+ mph. Ideas?

 

Did you replace all the vacuum lines under the hood? Sounds like the line to the vacuum reservoir is leaking...but all lines under the hood should just be replaced as they rot-out in 3 to 4 years.

 

I've got the same issue but it's all the time. Also when blower is on low I can hear a hissing sound under the dash. I've replaced the upper mode door actuator, but I think the lower mode door actuator is the one out now. I cAnt find the part any where though. Any ideas?