There isn't a single "neutral switch" in the way some vehicles have a clearly identifiable component with that name. The 1989 Ford F-150's neutral safety switch (sometimes called a neutral start switch) is integrated into the transmission itself. It's a part of the transmission's internal workings and not a separate, easily accessible component you can see or replace without transmission work.
To clarify, its function is to prevent the engine from starting unless the transmission is in Park or Neutral. The switch is physically connected to the transmission's shift linkage. If you're having starting problems related to this, it's a job best left to a mechanic who can diagnose the issue within the transmission itself.
