4 options:
1. leaking thru the seam sealer where the a pillar meets the roof, down the a pillar and out the seam just inside of the door switch
2. corner of the cowl where the cowl cracks. major seam there that never was sealed very well until the 88 up trucks and even then it cracked.
3. that vent you mentioned because the factory used open cell foam that degrades rapidly and doesnt seal against water.
4. firewall seam, which is magnified if the cowl cavern where the wiper assembly is is full of leaves.