I had the same problem with my '00 SE. It had a small crack that let in water. I replaced the piece thing with a used one from ebay. BTW - I still have the old one if you want to try to fix it.
It is very easy to remove, just pop off the inside matting and I think it'* 4 nuts (it'* been a couple years) to remove and the piece comes off.
To find the leak, I'd try putting the whole piece under water and hope it bubbled. It'* probably too long for a bathtub, but a cheap kid'* pool should do the trick.