Unlikely the screen unless you can see it coming in - take off the a-post trims down each side and hose the car a good deal, and have someone inside to look for leaks. It'll either come in the top or sides - water can't run down the outside of the screen, then back up the inside, through the glue, then do a u bend and run down the inside of the bulkhead

Either that or the heater matrix is fucked and leaking water in through the footwell vents.