Same exact situation, I couldn't figure out why/how the condensation was appearing for the life of me (I kept thinking it was a bad front window installation or sunroof) and then my CEM went haywire and crapped out, which is what clued me into the heater core situation. Most likely you've got a leaking O-ring. In my situation, the plastic snap connector had popped off and only time/grit held the o-ring in place until, at some point, the vibration took it's toll and shredded the o-ring and caused about 1 gallon of fluid to slowly leak under my carpet. Only at the very end (when the horn was blaring on its own), was I able to find any external coolant in the car. Once I pulled up the carpet, there it was and I felt like a dumba$$ that I had waited so long.Noticed some condensation on the windshield passenger side
So do yourself a favor and pull the CEM and fix the o-rings now before the coolant fries it.