Passenger Side Floor Wet

I like your idea of using UV dye, sounds like you're making progress on the diagnosis. Just wanted to add that I fought a similar leak a few years ago, and water would pool in that corner, and it ended up being a leaking windshield seal. My approach to tracking it down was stuffing paper towels everywhere I could in the interior and checking which ones were wet after a rain storm. Doing that I was able to determine that the leak was coming from the upper edge of the windshield.

 

It can possibly be it. I had the windshield replaced, along side the driver's window regulator. But so far they did a bad job with the regulator as the window is vibrating each time it goes up or down, and it is slow .

 

If you have a small handheld mirror, I'd suggest angling it so that you can see up behind the AC blower unit. Shine a flashlight up there and take a look. Up at the seam of the cowl you'll see some white putty-like stuff and it'll probably be wet if it rained recently. Like others have stated, you'll probably end up having to do the TSB. Also check your driver's side as it tends to have the same issue. I ended up having to remove the whole AC blower unit and box to get my hand up high enough to fill the holes with flex-seal. It's a weekend's worth of work.

 

Just saw the solution to this on 1st Gen Tacomas Facebook! Underneath the passenger side fender. Here is the picture explaining