The neutral safety switch on a 2000 Chevy Impala is located on the
transmission. More specifically, it's usually on the
side of the transmission case, near where the shift linkage connects. It's a small, usually cylindrical, switch. However, the exact location can vary slightly depending on the specific transmission (automatic transmission type) in your Impala.
Because of its location and potential for needing specialized tools for access, it's best to consult a repair manual specific to your Impala's year and transmission type for the precise location and any specific instructions. A diagram in the repair manual will be invaluable.
