Replacement neutral saftey switch

What year is the transmission? If it's an early '68 or older transmission, they used different internal parts to run it's single wire switch. You can't just swap in a 3 wire switch in place of a 1 wire. The "rooster comb" inside is different too. If it's a late '68 or newer transmission, it time to pull the pan to see what going on in there. On a 3 wire switch, the center pin is for the neutral safety circuit. It just grounds the terminal when the transmission is in neutral or park. That allows the starter relay to work. In the reverse position, the outer two pin are connected internally to complete the BU light circuit.