Before you bury yourself in those threads..... there are 2 high probability places to look first.
There is chrome trim above your front and back doors. It is shaped and acts like a rain gutter. If you look down into it, you may see that areas of the "gutter" have come away from the roof. A 1/16 of an inch is all you need, and water can get in. It will collect down your middle (B) column and accumulate on the floor behind the driver seat. Use some RTV silicone to fill any and all tiny gaps in that gutter.
Next, open your front and back door. Look at the rubber gasket on the door openings. Is it absolutely secured or are there areas where it has pulled away. Particularly look in the upper right corner. The rubber gaskets shrink. Again, your trim will nicely collect the water and deposit it on floor. You can RTV the gasket back or re-lay the rubber. Make sure there is no gap in the rubber at the bottom of the doorway. Also check for any cracks in the black trim around the doorways.
Check these two before you go towards the more esoteric possibilities. People dump their Bonnevilles because they have these exact water problems. They can be solved in 15 minutes to a couple of hours.
Welcome to the Bonneville Club, where not every answer is easy. But many just might be.
1995 SE ..Green and Rattle Can...234K mi...Bonnie (she'll outlive me)
1998 SSEI .. White ..119K mi...Evey
1996 SSE ..MajTeal..157K mi...4/12- Dealer trade full of Lucas Trans fix