I have the same problem w/my 5th gen...I recently took apart the dash trim and painted it a jet glossy black to match my car paint and when I went to put the trim back on I tried to work the AC but the cable seemed to be getting held up and the knob wouldnt turn past like halfway into the blue (this was over the summer)...and come to find out a couple days ago when It started getting cold I tried to turn the knob all the way to red and it would, but no hot air would blow out!

Would this be a prob with the heater core? If it is, how can I check/fix it? Thanks for any suggestions.