01 Ram 3500 4wd v10 206K miles. Ok, one of my worst fears. Yesterday started up truck for first time in a couple months. I turn on the fan to high, and defrost. My wife says, "what's that?" Damn! There are a couple of fresh rivulets of water on the floor mat, right top side of tunnel. This accompanied by an instantly fogged winshield. I'm thinking there went the heater core. This is probably my worst fear, replacing the heater core. How bad is it going to be. Any good tips. A blow-by-blow would be most helpful. The last time I tried this I broke the heater box trying to remove it from my 79 300.