If it was a heater core the liquid would be antifreeze. Very distinct smell and texture not easily mistaken for water.
It could be the well known water leak, or your cowl drains are clogged with leaf debris. Clogged drains would allow water to slosh around in the tray and enter thru the cabin filter or someplace else in the cowl.
Xcharger with water/meth injection, JBA headers
JBA duals with an H pipe, Roush springs, Che K member brace with torque limiters, Cervini's quarter window deletes, Canton polished tanks, Moroso fuse and master cyl covers, Cervini's cowl hood, custom GT500 chin spoiler, PJ rear louvers, Black rear spoiler, BSM custom stripes, Hurst shifter, Billet interior trim, custom door welts, Autometer dash pod, AEM wideband, Autometer boost & Fuel pressure gauges.