Fluid leak inside car

Okay, interior is out and I have a bare floor pan. Left the car outside last night (usually garaged) and there was a light rain. Moisture on both the driver's & passenger side front floor pan as if it was running down off of the trans tunnel. Unfortunately wherever it was running down from,I couldn't see as it had dried. I took a sec and blew out the heater core and there was still quite a bit of coolant in there, so it looks like I still have two possible culprits; bad seal somewhere under dash (windshield?) or heater core. I'm hoping it's the latter since i still had the leak when garaged & it hasnt seen rain in ages.