Determining if something is on the Left or Right of an Object

How can I determine whether something in the world is on the left or right of an object even if the object is rotations. I have a vehicle that i’m trying to create an AI for but i need to be able to determine whether the next waypoint is on the left or right taking into account that the vehicle could be in any rotation. I can’t use lootAt or similar functions.

By using this: http://forum.unity3d.com/threads/31420-Left-Right-test-function