So while rebuilding a steering column I found this plug. It's not part of the transponder ring that goes around the key it's a separate micro switch in the top of the barrel. If you unplug it the car starts and runs fine. What does it do?

 

Illumination ring? Or possibly the left key in ignition warning

 

ring for immobilser for chip in key

 

I know the black ring is for the transponder chip. It has wires which attach to the transponder box under the steering wheel.

But the white plug at the top of my pic is just plugged into the main loom. It's not illumination as there isn't any on a 99/00 classic. Thanks for the guesses so far :-)