The heater core is more on the passengers side of the vehicle, plus if it was leaking you'd have a problem with the coolant level and it usually smell up the car , plus you've have no real proper heat.
Most leaks are related to the windshield seal, or the trim around the windshield or from the trim that goes around the door or from the rubber that goes around the door.I have seen leaks that are coming straight down the trim and going right into the far right side of the drivers floor area etc, also I've seen leaks from the rubber seal plugs they use under the hood and on the doors etc.
Hope that helps and best of luck.By the way a good body or glass shop should be able to spot the leaking area for you.Plus if it's dry out and warm you could try to pour water on certain parts of the windshield or trim etc and see after a bit of time if any water appears in the vehicle.
Merry Xmas