It's the heater core, and since the whole dash has to come out, add some dye to your AC system so that you can see if your evaporator is leaking. Might as well do both at the same time since evaps are so problematic, it'll only add about an hour, the cost of the evaporator, and the cost of the drier. My cost on a heater core is about $160.00; my cost on the evap is about the same. Usual labor charge is 9 hours, with the evap at the same time, 10 hours.

Hope this helps,
Chris