1.  Wires caught in drive shaft.  It sounds unlikely to go unnoticed but I've repeatedly seen trucks where wires hanging down in the drive shaft have grounded to the shaft.  In these cases, putting the truck in reverse will often blow fuses.  Shorting to the drive shaft occurs in tow trucks because the cable ties, which hold wires in the body above the drive shaft, sometimes break, and because the drive shaft is not as easy to see as it is on other types of commercial trucks.  Prevent this problem by using heavy cable ties and clamps, and a lot of them. 

2.  Wires smashed inside toolbox.  This is one of the most common short circuits in conventional wreckers with tunnel toolboxes.  Frame forks, dolly axles and other heavy, loose hardware inside the toolbox will abrade and chop up the marker light wires that run inside the box.  While brackets to hold the hardware will help solve this problem, the ultimate solution is to weld in thick steel guards around the wires.

3.  Conductive or inflexible sheath.  Truck equipment moves and flexes, and thus requires non-conductive, flexible sheathing material around the wires.  Use plastic wire loom or some other rubber-like material.  Running wires inside metallic tubing often results in short circuits.  Inflexible sheathing material eventually breaks and creates a sharp edge at the break, which will cut through the wire's insulation and short out (see photograph above).  

4.  Squeezed by toolbox mounts.  Occasionally a tow truck body installer will accidentally fail to get an electrical cable out of the way when bolting toolboxes to the truck frame with an impact wrench.  The electrical cable gets sandwiched between toolbox and frame, often resulting in cross-functioning.  For example, the turn signals will come on when you turn on the beacon lights.  

5.  Radio.  The lowly radio is one of the most common culprits in tow truck short circuits.  Radios usually come with many excess unused wires, which the radio manufacturer has partially stripped for easy installation.  If the radio installer has not closed off those stripped wires, the bare end of the wire can intermittently short against the steel radio receptacle in the dash.  When the lights dim as you go over a bump driving down the road, check the radio.