Mystery Wire ? Is Something Missing ?
 

Checking my rear heat shield today I noticed a cable under the offside rear of my ZT. Its not the ABS sensor as that is the black cable at the top of the image and its not the Brake Wear Sensor either. The white plug is just behind the wheelarch on the bottom of the car and the cable disappears up behind the wheel arch. Any ideas ?

That could be the plug for the auto headlight beam adjuster found on models with Xenons. (assuming you don't have them. If you do, then I'm wrong :P )

Hmm I don't have them so possibly, would that be at the back though ?

Yes. The height adjustment works from a switch mounted on the rear arm. The heavier the load, the lower the car and the further the switch is depressed :}

I dont know what it is either............but my car has one too !!!! LOL