Where is the neutral safety switch?

The clutch safety switch should be on top of the clutch pedal.

If you have a 5-speed manual transmission, you're probably looking for the clutch safety switch, which is located on the clutch pedal under the dash near the firewall, rather than the neutral safety switch, which is on the left side of automatic transmissions. Out of curiosity, why do you believe that it needs to be serviced?

Its a 3.9 V6 by the way.
If its an automatic trans, have you checked the neutral safety switch ? Will it crank in neutral ?
Are you getting power on the small wire at the starter solenoid coming from the key switch ?

A mechanic would check for voltage at the starter solenoid, the neutral safety switch, and the key switch.
The light on the key switch should go off after you open and close the driver's door.

A mechanic would check for voltage from the key switch at the starter solenoid. If the starter will jump and crank, it could be the neutral safety switch or the key switch, or a problem with the battery cables.

Safety Switch Most neutral safety switches are combinations of a neutral safety switch and a backup lamp switch. Others have separate units for these two distinct functions. When the reverse light switch is not part of the neutral safety switch, it seldom needs adjustment, as it is directly activated by the transmission or the gear selector linkage.
Typical location of a neutral safety switch on a transaxle. Reprinted with permission of Ford Motor Company. A neutral safety switch allows current to pass from the starter switch to the starter when the lever is placed in the P or N position. Some neutral safety switches are nonadjustable. If these prevent starting in the P or N position and the gear selector linkage is correctly adjusted, they should be replaced.
To adjust a typical neutral safety switch:

• Place the shift lever in neutral.
• Loosen the attaching bolts for the switch.
• Using the angling pin, move the switch until the pin falls into the hole in its rotor.
• Tighten the attaching bolts.
• Recheck the switch for continuity. If no voltage is present, replace the switch.
• The switch sometimes controls the operation of the back-up lights also. With the engine off and the key on, put the selector in reverse position. Check the back-up lights to see that they are on. Then, check in the drive position to see that they are out.

It is screwed into the side of the transmission, it has a three wire connector going to it. See picture of switch below.

Replace the Neutral Safety Switch, it is located by the shifter.

try replacing the neutral safety switch