So your cord unplugs from the heater presumably (by your post)?
Unplug the cord form the heater, plug cord into power, test terminals on the end. If that's good, then it's the heating element.
So you're sure it's not working? Felt the heater/hose/engine at the heater after being plugged in and it's not warm, correct?
Asking because looking at the recent weather in Cincinnati, it hasn't been cold enough to even remotely need the block heater to aid in starting.
And your truck is not new. Are you sure the glow plugs are all working? More likely the problem than the coolant heater if it's starting hard.