If the water doesn't have an antifreeze odor, it is probably from stopped-up cowl vent drains. My mom had this problem for months (rainy season and car washes). We pulled the wipers and cowl panel, removed the debris (leaves, straw, etc), rinsed well, and have since had no more 'mystery' water in the passenger floor.
If it has an antifreeze odor/taste, it's probably a faulty heater core.
Hope this helps.
Sunday, March 2nd, 2008 AT 6:46 PM