How to fix/seal plastic shower stall

I've got one of those pre-fab steam shower enclosures. Its got a rectangular shower pan with curved corners, on top of which sits three of those hard plastic sides. The long side has developed a "leak" of sorts. Its not so much a leak, but I've tracked it down to when the water droples run down that side, they wind up curving under the side before hitting something with some brown color in it, then curving back out into the shower. There's also a little bit of a "shelf" formed about the rounded corner where water can collect, making things a lot worse. I've got limited access to the interior wall behind the shower and it does not appear that any water is leaking "out" of the shower - merely that it is travelling from the inside to that point in between the seams where there is some rust or something. The result of this is that about a half houw after every shower, I have a puddle of ugly brown liquid on the shower floor.
I have tried re-grouting the seam, but it doesn't work. It appears there is enough water gathering at that seam that it "pushes through" any grout I put on the interior side. I thought sealant would actually seal things, but it doesn't appear to be a completely waterproof seal - maybe enough to stop an occasional splash but not enough to push back direct water contact over a 30 minute period.
I've been using normal tub and shower sealants, which are all flexible. But in my opnion, they may be too flexible, allowing water to seap through them. Is there any type of sealant out there that really does form a waterproof seal in these seams. Something that is made to withdstand more direct water contact than the typical tub and shower sealant? Any other suggestions? I'd rather not have to rip out the entire enclosure or anything that drastic.
Thanks in advance
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