Due to the genius design of the dash you have to do the following, I have not done this so it is guess based on the workshop manuals I have, talking to the dealer and removing some of the interior myself:
1) Remove pretty much all lower dash panels in front of the driver
2) remove or at least move seats to access center tunnel bolts
3) remove center tunnel panels including the center storage box and unbolt the center tunnel
4) slide center tunnel back to gain access to lower dash bolts
5) unscrew and detach the center button box
6) Finally remove the side cover for the lower center dash
7) After doing this you may or may not be able to confirm where the water is coming from