In my line of work, dropping and hooking multiple container chassis every day, it is common to run into problems that other drivers leave behind for me to find and fix. This is a part of the job that I have grown used to. What throws me off is when something happens to my truck that makes the trailer not perform 
the way it is supposed to. There is a common problem with electrical shorts in some of the chassis that I hook up to that often results in a blown 30-amp fuse in the truck for the marker lights on the chassis. This being the case, you can say I have small stockpile of these handy for just such a problem.
During a routine pre-trip on a chassis like any other at zero-dark-thirty a few days ago, I came across one of these electrical shorts. It usually only happens when you hook up with your lights already on, which I usually avoid doing, but in this instance I had forgot to shut off the tractor lights prior to hooking the electrical cord to the chassis. Straight to the fuse box in the engine compartment I went with my fuse puller and 30-amp fuse in hand! This time was different though. As I pulled the fuse and shined my flashlight through it to see if it was in fact blown, it was still intact and good. Simple fixes like this do not often puzzle me, but I was even more perplexed as I ran through all the fuses on the panel with my tester, with all of them having the same result...not blown!
I sought to the Team Run Smart "Truck Smart" forum to see if anyone had any similar experiences I could learn from, but no luck from anyone of our members. It was later that day it dawned on me to follow the electrical lines from the battery box, which on my CNG truck is located under the passenger jump seat. This meant diving under the truck when the engine had cooled down and running a systematic trace of my electrical system to find the problem. Having almost given up, finding no loose wires or connectors, I gave one more shine of my flashlight down through the frame between where the cab is and where the 
CNG backpack tank is mounted. I'm glad I did because it was then I saw a little black box that resembled a fuse box, under the CNG tank and up against the left frame rail. With no top access to this because of the CNG tank, I took to it with a mirror, flashlight and a set of needle-nose pliers.
Sure enough, once I opened the top and got a mirror under there, I saw a secondary fuse box for the lights I never knew existed. Inside was another 30-amp fuse for the same set of trailer marker lights as the fuse box in the engine compartment, but this one was black and blown inside once I pulled it out with my pliers. This was my lesson in truck electrical for the day and I learned a valuable lesson on not giving up, even if there seems to be no obvious solution. Start at the source, trace your steps through the problem, and a solution will often find its way to the surface through perseverance.