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I suspect this is a 1st Gen, but you do not specify. I am thoroughly familiar w/ 2nd Gen Cowls, Cowl Drains, and front Cab sealing at this point, but not 1st Gen. As clearly shown in my extremely verbose thread below, on the 2nd Gens, sealer failure allowing Cowl water to enter the engine bay would not contribute to leaks into the interior unless there was a secondary leak(s). My 2nd Gen daily driver Cowl has been leaking to the engine bay for a decade or so. In your case, it is possible the water is traveling behind the sealer for a significant distance. If you remove the sealer on the inside till you no longer see rust, you will likely be able to figure it out. On my daily driver 2nd Gen, the bottom of the firewall behind the R wheelhouse rotted out. The source of that leak was more obvious than yours, apparently.

Interior cab rust near front right corner (Cowl drain)

 

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In my last pic it was apparent the water was getting past the seam sealer in the troth as soon as I seen those 2 streams rolling down the firewall in the engine bay.
Shortly after those streams were visible the inside started seeping out.
Now I want to know how it’s traveling from the troth and getting under the seam in the interior.

Possibly only indirectly related, as soon after you place water in the Cowl Plenum, the water will also likely be at the drains behind the fenders, if design is similar to 2nd Gen.

On 2nd Gens, AFAIK, the leaks tend to occur at the extreme R & L edges of the Cowl Plenum, in the vicinity of the drains. This is especially true at the R edge by the Air Intake Assembly.