I have a 99 Legacy outback. The heater controls are sometimes slow to change. Like when I switch from defrost to floor sometimes it will switch right away sometimes it can take up to a half an hour. Where should I start to diagnose this problem?
On this age of car the controls are vacuum operated. That means on the back of the heater control, plastic tubes go from the control to the various doors that open and close to send the heat where you want it to go. When sufficient vacuum is created at that particular door, the door swings open or closed.
In your case, either a tube has a leak (a small split), the control switch has a leak, or the particular door has a leak (or is gummed up with dirt and can't move freely) in its mechanism.
The only way to fix the issue is to start pulling interior panels, pull the HVAC control from the dash and dig into it. You may need to replace a part or two (go used from a junk yard), or just clean up the mechanism.
Find a friend who is mechanically inclined and feed him some beer, or bite the bullet and take the car to an independent garage, rather than a dealership. The fix could be a twenty minute job or 4 hours. Good Luck!