Funny this - i had an RT about a year ago.
I had EXACTLY the same problem! However, i couldnt for the life of me figure out where it was coming from! Still wondering to this day!
Now the thing with this is that i didnt have any problems at all - until i got the car re-sprayed.
Now when i got it re-sprayed, i took off the rear wiper, therefore had to plug up the end of the wiper jet pipe with a screw and some sealer.
What rear footwell is it in? Mine was in the passenger side - which coincidentally is the side my rear washer jet pipe was running down the side of the car!
Now what i figured, is that because i sealed the end up good and proper, if i were to accidentally knock the jet on on the back (which i did many a time), the pressure mightve been too much for the pipe somewhere by the side of the car, therefore leaking water down.
Thats the only thing i can think of to tell the truth, as i was pretty confused by it myself!
Hope this helps - let us know when, or should i say IF, you find out where its come from!
Al