It is a neutral switch.
Starters on newer Isuzu will have a safety-switch installed, even for manual transmissions. The switch, inside the transmission or clutch, will disable the starter circuit until the vehicle is in the proper "mode" to engage the starter.
For automatics, this means the starter won't engage unless the transmission is in Park or Neutral.
For manuals, (stick-shifts) this means you must engage the clutch while turning the key. Until you engage the clutch, it will seem like the starter is dead.
You can have a look at the related link for some trouble shooting pointers.
The safety-switch(called a Mode switch for AT) is on the side of the transmission and the clutch switch is on a bracket in front of the clutch pedal. It is activated (switches on) once the clutch pedal is depressed.
Does the oil pressure gauge move when you crank the engine? If not suspect bad oil pressure switch. Have you checked all fuses? Consider possibly bad distributor pickup or dead HEI ignition module.