Water leaking from under dash near the brake pedal.

[dkraven]

1977 GMC Sierra Grande, after a heavy rain is always water dripping down the rubber flooring to the left of the brake pedal. Is this more likely coming in at the window seal or one of these openings where the hood meets up?

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[GTX63]

Could be one or both, or neither. Start by removing the black plastic cover at the top of your second picture. It runs between and beneath the wiper blades and is known to collect debris over time and hold moisture. I recently had a 74 C10 I had to clean out first by removing handfuls of leaves, pine needles and mud, then shop vaccing it and then blowing it out with an air hose. It will rot out the firewall if left long enough.
You can also take a hooked pick and pull back your windshield's weather stripping, first around the drivers bottom corner, and look for rust. It may be dirt, but there appears to be discoloration around that area in your bottom photo.