What you got there is what we call a hot mess!
Just a reminder, (Don't ask how I know.) that is a one of two of your starter solenoids, it feeds power to your starter soleniod (2nd one.) that is on your starter. Yes you have two starter solenoids. Not sure if Ford was trying to keep the truck wiring and other things so they worked between a gasser or diesel, or if as others have suggested, the starter soleniod just takes serious power to energize it because its a diesel. But the one on the firewall is energized when you turn your key to start.
As Cubey shows you might have a older glow plug relay, which also looks like a starter relay.
Can you send a picture zoomed out, it is hard to figure out where this relay is and what it is based on your picture. But it appears to be the starter relay, but we don't want to guess.
So lets talk about the starter relay on the fender well. (Seems to be more common it fails.) You have two big posts and one or two little posts.
1) One little post energizes the solenoid and makes it click and pass power from one post to the other while you put the key in the start position. If you have a second post it gets 12v power when the soleniod is energized. (It gets 12v positive like the other terminal for as long as you keep the key in the start position.)
2) If that is your starter relay in the pic, your starter feed wire is not currently on the small post. So when you put the wires on the big posts it will not start.
3) One big post will have a positive battery cable hooked to it. Many other items will also hook that same post, its so they get 12v power to them. (Cleaner than hooking a bunch of wires to your battery.) But generally speaking they are only hooked to that side that the battery is hooked to.
4) The other big post is the one that will energize with 12v positive when you energize the small post, in this case when the key is in the start positon. It will basically connect the two posts together. Now in a gasser your starter motor will get that power but in these diesels it will only power your second starter soleniod that is on the starter. That solenoid will now connect the batter cable on the starter to power the starter motor. (Its redundant in a way and little odd, but it works.)
5) So the only thing that is usually on the second big post is the cable from the solenoid to the starter solenoid. Anything else you hook here will only get power when your key is in the start position. Now with that said, I have a 7.3, perhaps a 6.9 uses this for the glow plug relay or something else. I will let others chime in. But generally there is not a need for temp power, so likely all your wires except for the starter wire need to go on the one post with your battery connection. (So they all get power from the battery.) This would include the connection from the alternator, so it can charge the battery.
I hope that helps you out.