Basically, you would want to start at the battery and work your way in with a volt meter to see where the break is. A couple of things come to mind, though. On a 73, if the fuse box isn't attached with that long screw that goes into the middle of it, you won't have any power.On earlier cars they were just mounted to the car, but starting in 71, they plugged into a connector, so, if it's just hangimg down loose, it won't be getting any power. Also, I believe that the main feed from the starter solenoid to the fuse box is a fuseable link. If that wire blows, you won't have any power. Hope that helps.