I had a new stereo installed in my 2005 Outback. After it was done, I have noticed I cannot get the heat to come on. When I turn the knob, there is no warm air. The AC works fine. I suspect something happened when they unhooked the cable under the dash, and now that valve/door is not opening. How big a job is this to get fixed? Winter's coming, and I'll need to get this done soon. Just wanted to be prepared if this is a more involved job or not.