Well at least you know the starter is working, on the bench that is.
In these situations it is best to take a fresh start, a reboot.
All power starts at the battery. Remove the battery and test it.
Will the battery in the MG start your truck?
Before placing the battery back in MG, clean the battery terminal clamps. Auto parts stores carry a cone shaped wire brush for this purpose, it works great.
Remove the battery grounding cable at the body, use a wire brush to clean the area where the bolt and battery cable eyelet contacts the car body. Reconnect grounding cable.
Reconnect the battery.
Now move on to the grounding strap under the car. Remove it and clean all contact points. Reconnect grounding strap.
Now try to start it and report back.