[shorod]

The leak when using the A/C sounds like a plugged drain for the evaporator core housing. The condensation on the evaporator core would normally drain outside the car. If the drain is plugged, the water has no where to go other than through a seam in the housing and into the cabin. This drain should be low on the passenger side firewall in the engine compartment. There will likely be a length of hose to guide the water below the level of the car's floor.

As for using an adhesive to hold the cowl down, you'd be better off determining why the cowl does not sit tight and fixing that. Many modern cars have a cabin air filter that is installed below that cowl. To replace it, the cowl needs to be removed. If your Taurus has one of these filters, you certainly won't want to glue the cowl down. This would also mean that the filter was likely changed once before and maybe some of the retainers broke or were lost and not replaced.

There was a technical service bulletin on leaks during rain storms. Search this forum for keywords and you'll probably find more details.

-Rod