The turn signal switch is very often misunderstood as to how it operates. The turn signal switch does more than operate your turn signals. It operates the horns, brake lights, and emergency flashers as well. For those who are electrically inclined or just would like to understand how the turn signals work, below is an explanation of the operation of the turn signals. When viewing the drawings, think of the turn signal switch oriented where the lever is pointing straight up and the switch is on it's side. As you can see, there are multiple points where age or corrosion may cause improper operation of any of the circuits.

Also notice that the circuit for the brake lights goes through the turn signal switch. The brighter bulb element is used for both the brake lights and the turn signals. This is how the circuit for the emergency flashers works, using the existing brake light circuit contained in the switch. It's important that the turn signal switch be in good condition for the brake lights to operate properly. If the switch is worn, sometimes the actual weight of the lever can make the switch break the circuit for the left brake light but not quite connect the flasher circuit for that turn signal. This happens often and can confuse many an owner or mechanic in trying to learn why the left brake light won't work properly. A new turn signal switch does wonders for the overall operation of turn signals, horns, brake lights and emergency flashers.

Here is a dissassembled turn signal switch. The cam plate had been changed at one time but this shows how the contacts are oriented in the switch.
NOTE: There are kits that allow you to just change the cam plate on an old switch. For proper operation, the turn signal cam has to be correctly alligned and tightly installed. If the cam is worn out and needs replacing, chances are the rest of the switch probably needs replacing also. It is recommended that the complete switch assembly be replaced when needed to ensure proper operation of all functions the turn signal provides.