How to read variable from parent object?

Edit: moved from comments, really more of an answer i guess

must be taking direct input off of the mouse position, in which case.. if you really want to, you could add a var and pass the difference of the tanks rotation to it..

 offset = currentTankRotation - originalTankRotation ;
 turret.rotation = mousePositionX + offset; //pseudocode

oh,and as for how to read the var,you could just say:

     var tank : Transform;
     tank = transform.parent;
 
    var  tankYRotation = tank.eulerAngles.y; //for example

once you have that, you can access other scripts you're after too, with

 var myScript = tank.gameObject.GetComponent(MyScript); // MyScript = the name of the script
     myScript.hitPoints = 100; // var (of type int) on myScript
     myScript.DoSomething(); // function on myScript

EDIT:

one good way would be to just use the distance of mouse movement to apply the rotation:

 var lastMousePos : float;
 var currentMousePos : float;
 
 function Start()
 {
 currentMousePos = Input.mousePosition.x;
 lastMousePos = Input.mousePosition.x;
 }
 
 function Update(){
 currentMousePos = Input.mousePosition.x;
 var thisFrameRotate = currentMousePos.x - lastMousePos.x;
 transform.Rotate(Vector3.up, thisFrameRotate);
 lastMousePos = Input.mousePosition.x;
 }

not tested but should work, at least to give an idea

and glad to help!