Bone rotation during animation?

Hey guys, after about an hour of trawling the forums/answers I can't find a reliable solution. Here are the details:

The problem:

My player is able to aim up,down,left and right with no problem whatsoever and I have successfully rotated the bone with no problem. However, if any animation is playing, the bone will refuse to rotate. I have tried implementing the rotation function into FixedUpdate() but that just snaps it back to the animation after 1 frame.

What I want:

I would like to know how I could achieve the effect of controlling, a potentially, animated bone during an animation. So I want to override the original bone animation with script. Is this possible?

My script for the sake of it:

 var spine : Transform;
 var rotateSpeed : int = 18;
 var pos : Quaternion;
 
 function Start()
 {
     animation["Idle"].layer = 1;     
     animation["Idle"].AddMixingTransform(spine);
 }
 
 function Update()
 {
     if (spine.transform.rotation.eulerAngles.z > 294)
     {
         Debug.Log("294");
         spine.transform.rotation.eulerAngles.z = 294;
     }
     
     if (spine.transform.rotation.eulerAngles.z < 244)
     {
         Debug.Log("244");
         spine.transform.rotation.eulerAngles.z = 244;
     }
     
     if (gunAiming.aimMode == 0)
     {
      spine.transform.rotation = pos;
     }    
 }
 
 function LateUpdate()
 {
     if (gunAiming.aimMode == 1)
     {
         spine.transform.Rotate(Vector3(0, 0, Input.GetAxis("Mouse Y")) * Time.deltaTime * rotateSpeed);
     }
 }

Please just ask if you need any more information, and please do not tell me any script, I like to learn not implement.