Cowl drains need to be cleaned periodically because rotting leaves, dirt, insects, & fungus build up in them. This shows one with the fender removed, but they can be accessed by removing a few of the 7/32" (5.5mm) screws along the lip of the wheelwell just behind the front tire. Or if the wiper valance is removed (or through the antenna hole), they can sometimes be blasted out from above with a garden hose, but on the R side, it may splash mud into the HVAC blower.
To prevent reoccurrence, add screens to the fresh air intake under the wiper valance. To keep leaves out of the passenger side (where the HVAC intake is) and force them to the unused driver's-side drain, add screen at the top, just below the wiper valance:
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To keep leaves out of the ventilation system, add screen inside the cowl by removing the wiper valance & working through the rubber plug under the antenna:
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