Water on Rear Passanger Floorboard 03' Buick Park Avenue Ultra

About 3 weeks ago I started having water show up
on the rear passanger floorboard (and only there:
none in the front floorboard or the trunk). At frist I
thought it might be that one of my kids had not
shut the door all the way so I cleaned it out, put the
mats back, and made sure the door was closed.
But it showed up again a few days later (and it had
not rained during that time). So I cleaned it out
again and left the mats out just to keep an eye on
things. Well the next day I found the water was
back.

Looked around the internet, and found that some
people say it might be the AC drain being clogged
up.

My problem is that I have no clue where the drain
is located. I know there is one on the firewall up
front, but is there one for the back also? There are
vents back there so I am guessing there is one that
I can't find.

Can anyone help point it out for me?