I have a very similar problem to this post: http://www.2carpros.com/questions/1990-subaru-legacy-heater-only-blows-through-c-vents-push-defrost-button-air

My A/C controls are different from those referenced in the solution. It doesn't matter if the temperature is set to hot or cold, nothing comes out of the defroster vents. I suspect that the defroster door is just stuck. Is there a quick way to determine what the root of the problem is before I take it to the shop (or fix it myself, even though I'm not overly mechanical)?