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!

Well from what I gather from your post, you could do something like:

 var movementScript = transform.parent.GetComponent("Movement");
 turretAngle = movementScript.directionAngle;