I have the same leak 1999 NBS Ext Cab.
It leaks on the floor in the Passenger side front and travels down the
sill to the back floor and soaks that too. Not Antifreeze it is just plain water.
I do not know what to do about it either. It just started doing it one day when it rained real heavy for hours.
It screwed up my amp rack (blew all the fuses and destroyed one of my Signal cables) got lucky as my rack is raised and the amps were saved by 1"
PLEASE SOMEONE HELP US!