Starting

I have long had had a starting problem where it is necessary tp select Park/Neutral alternatively several times until car starts.
I have bought a new battery, had the starter motor and solenoid checked. I have recently had the camshaft sensor changed but without effect. My local vauxhall dealers have quoted £429. 60 to replace the selector mechanism but have also suggested that there is a neutral position sensor which they can supply for £ 36. However they are not able to tell me where this sensor fits. Does anyone know how to fit this sensor?