Hello,

Its called a transmission range sensor on the transmission near the front of the car, here is a diagram to show you where. On a manual transmission car the switch is up at the top of the clutch pedal. Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.

have a 04 pt crusier and my mechanic said he changed it and did not,so I am trying to change it and I need to know how.

If he charged you to replace it, I would take the vehicle back and make him do it. The switch is on the transmission housing. All you do is remove the wiring harness to it and it unbolts.