Zombie Damage Parts Problem

Hello everyone so I basically have this problem, which is kinda frustrating. I have created a health system for the enemy (in that case a zombie) and I want it to take damage differently, according to the area I hit it. In this case I want two areas: Head and Body. Problems occur when I try to do this by collision detection it's not even recognizing and separating the body from the head, although they are different meshes. Here is what I managed to do so far. Any help would be amazing!

 using UnityEngine;
 using System.Collections;
 
 public class EnemyHealthSystem : MonoBehaviour {
 
     public int maxHealth = 100;
     public int curHealth = 100;
     public GameObject zombie;
     void Awake () {
     
     }
     
     // Update is called once per frame
     void Update () {
         Death ();
     }
     void OnCollisionEnter (Collision col)
     {
         //This is the part that doesn't work. I want it to recognize the 
        // Object that is colliding on and subtract the number from the
       // current health of the enemy.

         if (col.gameObject.tag=="head")  

         {                                                       
             curHealth -= 100;                  
             Destroy (col.gameObject);
         }
         else
             curHealth -= 20;
             Destroy (col.gameObject);
 
     }
     void Death()
     {
         if (curHealth <= 0)
             Destroy (zombie);
     }
         
     }