Also, I can't remember if I needed to unbolt the rear (upper) seat back too ...to pull out the plastic window surround. Something in the back of my brain tells me I had to. But I don't remember 100%. Maybe there's a youtube video on the rear drains. Either way, it's not difficult to unbolt the rear (upper) seat back.
Also, that big plastic rear panel, has metal clips. I found just pulling outward and wiggling at the same time the clips come out pretty easy w/o breaking. The wiggling helps release them. I've had my entire H3's interior out 3 times...LOL including carpet and headliner (not the dash). It actually comes apart really easily once ya do it.
You might try parking your H3 uphill and pour some water into the rear drains. If they aren't backing up, then you probably don't need to mess with them and saved yourself some work. They're fully open tubes on mine (no fingers to clog).