The neutral safety switch is located on the steering column just above the toe pan. If you have an automatic, the switch also controls the backup lights. The switch is activated by moving the shift selector, which in addition to changing gears, also moves a rod (reverse lock out rod) which physically rotates the column tube and changes the switch position. It's a white piece that should have two connectors attached to it (if you have an automatic).
Why are you testing the neutral safety switch? Will the car crank in drive?
Now, if you have a manual transmission it's different. There is a clutch safety switch which won't allow the car to crank unless the clutch pedal is depressed. That's located on the pedal brace and it also a white part. That switch is very, very difficult to find if you need to buy a replacement. It yours doesn't work, soak it in simple green to clean it...cause they get dirty and lose connectivity.