Tried a search,but........

I noticed in my truck yesterday that there was water on the passenger side rubber floor mat. It has been raining hard off and on for the past two days. The rug was not wet (as far as I can remember). I am trying to determine if the water is coming from the windshield or underneath the glove box.

Anyway to determine this?

Is it possibly the heater core?
What makes them go bad?
What are signs of a bad heater core?

If it is the heater core, how much are they and are they easy to change?

My X has almost 45,000 miles on it.

Thoughts?

Thanks,

Stone

PS: I have had the defroster on for the past two days due to the dampness out and shut the truck off with the defroster still on. Does that matter?