The one that goes from the top of the engine (way in the back) to the heater control valve. It was really hard to see. I ended up finding it by just putting my hands where the steam was coming from and reaching around like Ray Charles. Glad I didn't get burned. I'm so glad the weather was nice today. Around this time it can get pretty cold and nasty on the east coast.
Oh yeah, found out that my "check coolant" sensor doesn't work anymore. Great.