Assigning target to send message to prefab in Inspector

I want to make an enemy prefab which would send a message to player when gets destroyed. The problem is that I cannot assign objects from scene to prefab slots in Inspector. FindWithTag function is not working either...

public var lives = 25;
public var blood : GameObject;
var t : MonoBehaviour; //Target of the message to be sent 
var message : String ; //Message to be sent

function Start(){
 var t = GameObject.FindWithTag("PlayerTag"); 
}   
   function Update (){
   if(lives <=0){
  dead = true; 
  Destroy(gameObject);
            Instantiate (blood, transform.position, transform.rotation);

            t.Invoke(message, 0);

           // iTweenEvent.GetEvent(GameObject.FindWithTag("PlayerTag"), "Shake").Play();
  } 

   }

   function OnTriggerEnter ( other : Collider){
    if (other. tag == "bullet"){
     lives -=1;

     Destroy( other . gameObject); 
    }
   }

The 't' in Start() is local; it is not the member variable because you have re-declared a new one with 'var'. So you are never actually assigning anything to the member variable 't', so presumably you're getting a null reference exception when you get to t.Invoke.

Also note that the member variable 't' is declared as type MonoBehaviour, but FindWithTag will return a GameObject.