If your sure it's water then check the cowl box at the base of the windsheild , If you lift the hood you will see a seam that runs accoss the fire wall. The seam forms the cowel box. Sometimes the sealent used in that seam will fail where it's up under the ACbox and it will drip into the heater box on the inside and drip behind the glove box. If the drain is plugged up on the A/C you can get a dip in the same area. A bad heater core will also drip there.
If you pull the windsheild wiper ares off and under the hood will be a few screws and then you can get the grill peice of and get into the cowel box. You can use some RTV to seal it.
Could also be the windshield gasket.
You should try to isolate the area with a waterhose to make certain.