Won't start- please Help

[Theoriginalsilverado]

Quick question. Hooked everything up to my motor to make it run today after putting on the newly painted cab. Motor turns over with turn of the key but motor will not catch. Any idea what might be missing? There is one wire now not hooked up that I think could be the cause. I don't have any pictures so I found another similar engine bay online for reference.

I've circled the connection in question. The red wire that comes from the junction and runs to a fused connection, does anyone know what that connection is for? I have not hooked it up but I realize now that it may be a crucial link? Or not?

This has me super frustrated so any help would be greatly appreciated