I joined this forum after lurking for the past year specifically to reply to this post. I also had NO heat or defrost for over a year. I bought the truck in July of 2012 and found out the hard way I had no heat (around November). I did hours and hours of research to decide that surely my heater core was bad. I ran water via a garden hose through the core in/outlets followed by compressed air. I noticed that "not much" water was going in before it began to come out. I determined that my core must be severely plugged. I roughed it out through the Indiana winter determined to change the core when it warmed up. I waited until last month and finally tackled it! There's a nice youtube video that covers about 85% of the job. Man was I excited once it was all back together with only 2 extra screws (out of probably 80 or so - I'll take it)... Well, I still didn't have heat. I did everything as directed to bleed the system properly. Hmmm well I ran a flush and that didn't fix it, so I know I didn't have huge deposits in there. I pulled the inlet hose off and started the truck. Nothing. I determined that my water pump must be bad. Mind you, I had NO coolant leaks OR overheating. I wish I had saved the picture, but the fins on my old pump WERE INEXISTENT, and the impeller was literally as thick as maybe... 3 sheets of notebook paper. Changed the water pump and once burped free of air I have awesome heat. Not to mention, my passenger side head gasket leak DISAPPEARED. I have a 4.6 Windsor and had planned on doing the head gaskets but decided it wasn't worth the money for the. $4000 I bought the truck for (109,500 mi - 120,000 now). It's been a month since doing this, the oil was changed and has the full six quarts. I've not lost a drop on my garage floor and the familiar burning smell / smoke cloud is GONE. I know this post was cumbersome, but if wanted to achieve a lot of information on one shot. I hope this helps at least one person because I found exactly zero people with this experience online! Good luck and PM me or email me with questions (anyone).