I don't know that particular unit, but...
Generally speaking the FPCM is a control-unit that modulates the fuel pump motor speed to control fuel pressure.
The output from the FPCM to the motor is pulse-width modulated to control speed.
There's a fuel-pressure sensor at the engine which provides feedback to the unit.
When the pressure is lower, the FPCM runs the pump motor faster to increase pressure.
When the pressure is higher the FPCM runs the pump motor slower.
Why the hell they would have thought to mount that unit under the chassis is beyond me. It doesn't even need to be near the fuel-tank. Under the driver's seat would work just as well.
The only place on the truck that would be more stupid than under the chassis would be in the fuel tank, or maybe bolted to the side of the exhaust manifold.
Brad,
I'm pretty sure that if you turn the key to ON, but don't hit the starter, the pump should run for about one second. You should be able to hear it.
You may need to have someone turn the key for you so you can stand beside the truck near the tank and listen..
