R/LG wire is key power - 12 volts.
The dash-ed wire goes to the PCM.
It grounds it when it wants GPR to activate.

30A fuse is fuse #22. If the fuel heat shorts, quite common, then it blows that fuse. That fuse also controls power to the PCM. Truck will never start is blown. If it is blown then try unplugging the fuel heater wire located on the drivers side of the fuel filter canister, second wire from top. It is hard to get off because it is a spade connector, like a speaker wire. The heater is really not needed but can be repaired if you want to.

One of the large wires on the GPR goes to the starter solenoid and is hot all the time. The other large lug should have two wires on it and each goes down to the valve cover connectors. As you can see the outside connectors on each plug supplies voltage to two glow plugs. Pins B and D control two injectors and pin C is injector ground.

The box you are referring to is the IDM. The PCM is located further down and most of it is located behind the emergency brake kick panel.