What's it taste & smell like ? If it's coolant it's the rear heater core leaking. Are you losing coolant? If it's just water, start at the top with the roof rails. Pull 'em off and look for roof rust holes. Crawl underneath with a flashlight and look for holes or missing floor plugs. It's tedious process. You may have to pull interior trim off and look for a water trail. It can be a lot of work. That's why shops get a lot of money to fix it.
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