Only when its raining?
Pull the drivers side carpet up and see if its got a trail of water coming in after a drive (in the rain) from the large pipe just to the right of the accelerator pedal which goes out into the wing
If its in the dry then have a little taste on your finger, water or coolant? aslong as your not allergic 
If its under the top rail then get a buddy and pour some water and check, if no joy then head to the jet wash and point and shoot one area at a time (time between areas for it to run)