I just bought a 2000 GMC 1500 3 weeks ago, heater worked great. I had surgery on my ankle and the truck sat for 2 weeks while I was on crutches. Well my wife took the truck to get straw the other day and about melted the heater was working great, the next day I drove it same thing. Drove to work yesterday and was very chilly, I could only feel heat comming out while the blower was on number 2 setting. On the way home this morning just enough heat to defrost the windows. I've checked the heater hoses-both are warm, so I'm ruling out the core for right now. Any ideas?