The steering servo is like the main tendons of your body and in an RC toy, they push and pull the Servo Horn, which in turn moves the Steering Links, so the Wheels will turn Left or Right. It functions similarly to muscles responsible for expansion and compression. With the truck turned off, if you move the wheels from one side to the other and back, you should feel and hear the slight resistance to the Steering Servo. With a fully charged battery installed, put your trucks wheels all the way to one side or the other, then set it down and turn on your controller. Then turn on your truck, and when they Synchronize together, the wheels on your truck should snap to the center position. The Steering Trim button on the Controller should move the Wheels as well. This Servo has a 5 wire plug in to attach to the ESC/Receiver.
If you find your steering is not responding, this electronic unit may be in need of replacing.