The thick rubber seal at the base of the rear window just above the trunk. This usually results in water in the trunk.

Sunroof drains/seals usually cause water in the car (back seat floor)

i found the same problem today my black floor is soaked in water and it smells too! but im sure mines due to a very leaky trunk and when every i stop all the water rushed to the front of the car and gets caught under my back seat and some get past and get under my carpit. is this a commen problem with bonnevilles?

I had clogged sunroof drains. I unclogged them, but that was not enough. I also had to replace the sunroof seal. That solved it. However my symptoms were different - I had a lot of water around the headliner.

Well - I guess I will replace the rubber around the rear door and see if that takes care of it.
I let you guys know what happen'*.
Thanks for all the info...............

any updates? my 95 bonneville also leaks bad

I still have the car and still get water, but I don't drive it - my son does.
I never replaced the seals around right rear door. It did not leak for a long time so I just left it - but after driving thru a hard rain sure enough there was the water.
I do belive it is the seal. So if you are going to keep it think about a new seal - but do it yourself - $. If not - clean it and the door where it makes contact and hope it does not rain.
Sorry I can't help anymore then that.