Mirage develops scenic duck pond on rear floor in the rain

[81.Chevette]

My 2017 Mirage hatch came with a bonus, unlisted feature I thought I'd share with you guys.

When it rains, an incredible amount of perfectly good water appears on my rear passenger floor, at absolutely no cost to myself. Unfortunately, I can neither use my back seat as a relaxing park, nor can I relax and park the car when weather shows up. So I suppose I'm going to have to fix it.

I'm not sure when the problem started. I first noticed that the carpet under my WeatherTech mats was soaking wet last year, but as I drive Lyft I just assumed it was the cumulative water from the shoes of 50 passengers a day for an entire rainy week. This week, though, was only delivery for me and it's happened again. This time I noticed that the plastic cover protecting the seat belt from passenger feet was actually full of standing water, so it looks like it's getting in through the B pillar? But that doesn't make sense. I've been in no accidents, and the doors and windows close securely. It's not coming from the front passenger seat, so that rules out heater core and I guess it rules out the AC drain as well. There doesn't seem to be any path from the door seals to the inside of the B pillar, though, so I'm not sure WTF is going on.

Ideas?