Somewhat common, I've repaired several, not too hard to remove from the bottom if you remove the starter motor first. Most likely the part you need from Cummins is heater element 3907766-s $35.18 or cable harness 3843722-s $49.39 depending on where it's leaking from. If the heater element breaks or gets a hole in it, fuel follows the wiring to the outside. If the wiring harness is melted then it's likely the problem.

The return hose can also leak onto the heater tricking you.