Wiper motor grommet and sealant pack. But this usually drips straight down.
With the glove compartment removed, you can spy the wiper mechanism over to the wiper spindles to check for leakage there.

Leaking "bulkhead" (black metal triangular inserts in the firewall, bolted in). Loosen the lower bolts you can reach and flow in some silicone sealant. Tighten up. But this doesn't explain why leaks stops when you cover the cowling.

Body seam crack someplace hidden in the cowling drain pathway? Rust hole?

Leaves and debris clogging the heater fresh air vent?
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1989 740Ti 1986 240DL 1984 244GL