Quick-Splice Connectors

Some brands of tow lights have quick-splice connectors inside the base to splice the wires together.  Quick-splice connectors are rectangular, usually blue in color, and sold under trade names like Scotch-Lok by 3M.  Although quick-splice connectors save the installer time because they don't require any wire stripping, they tend to make a loose connection over time, particularly when flexed repeatedly.  To solve the problem, get rid of all the quick-splice connectors in your tow lights and replace them with conventional tubular butt connectors.

Unseen, Twist-Induced Short Circuits

Being a creature of habit, I tend to walk the tow lights to the towed vehicle, then walk them back to the tow truck, and then coil up the electrical cable all in one direction (clockwise, for example), the same way every time.  Over time this repeated coiling activity can twist the four electrical wires inside the black outer cable jacket in the same way that you twist spaghetti when rotating a fork.  Eventually the wire insulation breaks and the bare wires short out against each other.  People tend not to believe they have this problem because the black outer jacket on the electrical cable remains unbroken while the individual wires inside have broken their insulation.  Fix this problem with new electrical cable or a new set of tow lights.

Worn-Out Pins on the Tow Light Plug

Each of the four pins on the female tow light plug receptacle -- that is, the part with the flip-up cover that's attached to the tow truck bed -- is actually split down the middle to provide springiness and make a good connection.  If the tow light plug has been yanked out at an angle too many times, the two halves of each split pin can be crushed together, making a loose connection.  Then the tow lights only work properly when you wiggle the male plug in the female receptacle.  Eliminate this problem by installing both a new female and a new male plug.  Out in the field, you can also temporarily fix this problem by taking a small, thin-bladed flathead screwdriver and gently inserting it between the two halves of the split pin to spread the halves slightly.