1980 300SD rain leak

I have a 1980 300SD that is in great shape, just under 100K miles. It's developed some sort of rain leak. If it sits in the rain for
awhile, when the heater blower first comes on, I can hear the fan spinning up in a pool of water. I have also seen some wetness under the glove compartment area.
My plan of attack is to remove the grill under the windshield wiper arms, as I believe that is the outside air intake area and it's probably getting in somehow from there.
Thought I'd ask in case anyone has any experience with this.
Thanks much.
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