Water leak in cab

1985 RC never been wrecked. Hey all. I have a water leak coming from up under the dash by the E-brake on the drivers side and by the passengers right foot. It leaks on both sides. The leaks appears to start far up in the dash beyond eye site and follows wires down to the floorboard. Initially I thought it was the windshield and/or seal. I had both replaced and had the seal trench inspected for rust. Everything checked out great but the leak still continued. I took the truck back to the windshield installer and he double-checked his work. Once again it checked out fine. He performed several water tests and claimed the body/firewall was leaking. At first I was suspicious but after further investigation I agree with his theory. I would like feedback and a reality check on this one. Beneath the front fenders where the firewall panel meets the side panel and windshield panel there is a gaping hole. It’s the location where all three panels meet and are pressed together but leave a small hole. From the factory they tar tape a piece of plastic over the hole. The problem is the plastic eventually becomes “unglued” exposing the hole. As water naturally drips through areas of the body panels some eventually make it down the hole and into the cab. I had a parts truck without fenders and after inspecting this problem area realized the potential for further leaks. This is the only way I can explain my leak and it appears as though it will only get worse as time goes by. Almost everything on my truck has been replaced or is brand new. I am now ready to trick out the interior but do not want to invest the time and money until the leak is fixed. It rains is Portland, Oregon 6 months out of the year. The only way I can think of fixing this issue is by pulling both fenders to properly access this water port. Do you think this is necessary? My second question is what do I use to plug the hole? Should I repeat the tar tape and plastic as the factory did? Silicon? Any suggestions would help. I prefer to do all my own work but would entertain paying a body man to fix this one. Thanks for your time.
Cliff out
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