I'm the new owner of a 2004 V6 with 170k mi on it currently. I'm also getting waterfall behind the dash so I went ahead and conducted a chemical combustion test today.
This test comes with dark blue liquid. If the liquid stays dark blue, your engine block is sealed and good. If it turns green, there's probably a slow leak. If it turns yellow, HG is blown.
Mine came out blue, kinda teal. That means I probably have a slow leak and will probably have a blown HG in a few years. I recommend everyone with a 03-05 V6 do this.
Currently I will do nothing and test again after maybe 8-10k mi. My plan is to add Blue Devil before it blows and I think that will do the trick.
Update to my post. My HG is now starting to leak. Coolant temps going up and down and radiator is bubbling from the cap. Leak tester shows same green color indicating slow leak.