I also forgot to mention, I also notice some steam coming out of my a/c vents, it happened for 2 days and it stoppedI'm not sure. I own the previous generation Mustang, so yours is a little different than my car. I'm pretty sure that is the condensation drain, but not 100%.
I think that if it was one of the heater core pipes, the coolant would be running out of it very rapidly.
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