Neutral gear safety switch

[David C]

My memory is letting me down. As Dodd says the reverse switch is on the shift lever and the neutral inhibit on the gearbox. A bit of research suggests that the culprit in your case is the lever inside the gearbox that operates the switch. This is known as a Rooster Tail due to its shape.
Be aware that there are at least three different types of switch and Rooster Tail. Pre about 1970 - per the 410 - the switch had a single contact and only inhibited the start. After this the switch had 3 wires and dual function to include reversing lights.
It seems it is possible to change the rooster tail with the transmission in the car but I can't immediately tell whether the valve body has to be removed.
You may want to consider a couple of upgrades while you are about it. The simplest is to swap out the pressure regulator valve for Sonnax 22771A-02K and the manual valve for Sonnax 22771-09. This allows the transmission to fill in Park and saves the "clunk" when engaging drive.
The other upgrade is to replace the valve body with a slightly newer unit to allow part throttle kickdown.