Coolant glows under UV light without dye. I had a small leak HG in cylinder 5 and was able to see the coolant when I shone a light down in it.

 

The a/c compressor also runs when you are using any variety of defrost. Refrigerated air carries less moisture. Although, so far as I am concerned, if I am running the defroster, ALL of the outside air is already refrigerated.

 

Had the same problem, water after rain. It turns out the frame rotted under the windshield to a poor glass installation. Had to pull the windshield, repair the sheet metal, and then reinstall.

 

It's to drain any water that gets into the bottom of the box. Whether it's from A/C condensation or the rain collecting in the air intake. You pull air in to cool it. Anywater that gets in rolls right out too.

 

I know it runs when the defrost is on. This is definitely an after the rain problem though.

Sounds like a lot of work there. Luckily, not much rusting here in GA.

Gonna pull up the carpet and run the hose soon as I get a chance. I've watched a few videos too. @HeyYou, I'll def check the back glass as I've had problems there in the past.