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the top is the return. Ford's don't have a check valve in the lines so you are ok and a cooler normally doesn't care which way the fluild is going through it. Slight chance you could dislodge some built up junk in the cooler but that's what the filter is for right?
I know this sounds stupid and messy but take a transmission line off. Have someone start the truck and see which way the fluid flows. As soon as you see which way it flows have them shut it down. It may work too with a starter button on the soleniod and just cranking it motor over. If I knew the answer I would sure tell you but unfortunately I don't, and this was the first thing that came to mind.
Thank you lord I've been searching for weeks never thought to check the expo forum.