How do you find an a broken wire in a harness?

For example. Let's say you have no brake lights. You've checked for power coming out of the brake light switch and you have power. But when you test at the rear light sockets, there is none. You've decided that the wire must be broken somewhere between the 2 points, but you don't know where.

You have a couple of choices. You can start mutilating and carving on the harness, pulling interior trim (and your hair) out trying to find the broken spot to fix it. Or, you can run your own wire (over-lay) from the switch to the rear light sockets which will be almost as much work for all the interior trim to remove to hide the wire.... which may still come out looking funny.

Is there a 3rd choice? Maybe. What if you knew within a few inches in the harness where the open was. Then you could go nearly straight to it, fix it, and do a neater job right?

Use your car's radio, a calculator (or anything else with a large LCD display). Tune the radio to a quiet AM channel in the low end of the band. Run a wire from the car's antenna to one end of the problem wire. The antenna must have a metal mast. You must have both ends of the wire disconnected in the car so that the wire is not connected at either end and just laying in the car basically.

Pass the device with the LCD display along the wire starting at the end you connected to the antenna to and listen to the radio. You'll hear the noise from the LCD coming over the car's speakers. When the noise stops, you've passed the break. The larger the display the better for penetrating plastic interior trim. This also helps in figuring out which way the wire goes in a harness that branches different ways and is all wrapped up tight in tape and loom without ripping it all open to look.