Stick shift transmission

On tow trucks with stick shift, the backup light switch is usually located on the transmission's top cover opposite the position of reverse on the shifter knob.  For instance, where reverse on the shifter knob is to the driver's side and forward, expect to find the backup light switch on the passenger's side and rearward, down on the transmission's top cover.  Because the backup light switch tends to be on the top of the transmission, on many trucks such as 1990s-era International 4700 and 4900, you can get easier access to the backup light switch from inside the cab through the floor panel, than you can from underneath the truck.

Automatic transmission

For purposes of backup light troubleshooting, I divide automatic-transmission-equipped trucks into two categories:  one in which the engine has to be running to activate the backup light circuit, and one in which the engine does not.  Some of the older Allison AT545 automatics, for example, can be put in reverse with the ignition on, but they will not energize the backup lights and backup alarm until you actually start the engine.  Many of the newer Allison 4000 series, on the other hand, will energize the backup lights and alarm without the engine running -- all you have to do is turn on the ignition and put the gear selector in reverse.

Why does it pay to think about which type of automatic you have?  Despite the piercing and sometimes annoying high-decibel volume of the backup alarm, it's often impossible on diesel-powered trucks to hear the noise of the backup alarm from inside the cab when the engine's running.  So a situation arises that tempts someone to be in back of the truck when the engine's running, and the truck is in reverse, and no one's in the cab.  It's not necessary to be run over by the tow truck just so you can check the backup alarm.  Do the safest thing and use two people:  one in the back of the truck looking at the backup lights and listening for the backup alarm, the other in the cab of the truck, running the engine at idle, putting the gear selector in reverse, and also applying the brakes so no one gets run over.

All backup light and backup alarm circuits, whether stick shift or automatic, work the same way:  current goes from the fuse block to the backup light switch on the transmission.  The backup light switch completes the circuit and sends current to the backup lights and backup alarm, both of which have local ground.  If the backup light and backup alarm have a good ground, and they come on when you give power to them, then look at whether you have power going to the backup light switch on the transmission.  If you have power into the backup light switch, and no power out when the appropriate action has been taken with the engine and the transmission in reverse, then install a new backup light switch.