Just like I've been working with Jason on his heat, or lack of heat, problem, it's a process of elimination. The same goes for why the ducts won't change for the defrost. Always start simple and progress to the complicated. You've already been told the possible culprits for the defrost. For the lack of heat, it could be thermostat stuck open, clogged lines or core, heater control valve inoperative, bad core. If you replaced the thermostat, the next likely culprit may be the heater control valve, inline in your heater hose near the firewall. These aren't fixed, but replaced. If you replace it, make sure you DO NOT reverse the vacuum lines. You should be able to get a replacement without much trouble, though don't be surprised if you have to wait a couple of days as it probably isn't a stock item at the parts store.