So when you have 12v power to both small wires you have no power to the second big terminal? The relay is shot then plain and simple.
A quick definitive test, take a knife or something metal and put it accross the big terminals for 30 seconds or so then jump down and start the truck. You'll know if it worked. Also whatever you used will discolour from the heat.
No, no & NO!
That's not how it works,,, Key ON, with either the GPs activated or Not, you'll see 12v on Both sm terminals. There's Always gonna be 12v on both of them because they are connected inside, to one another, by the coil windings. Anytime there's 12v on one it's on the other too, and you see that because YOU are supplying the path to Ground with your testing device (meter, light, probe, whatever, it has a ground connection - right?).
Test the GPR by allowing the 12v supply to flow to ground by grounding the side which is Not supplying 12v (you have to remove them from the relay to know which wire that is)
I don't know what kind of "knife" you would want to short a 150 draw with (might never be the same
) I would do this with a jumper cable! But, Again, to properly test the relay you confirm 12v supply, confirm activation by grounding, confirm the internal contacts are OK by measuring the voltage difference at each of the Lg Terminals when activated...