"How to make and display in GUI timer"

My car's inspector look like:

My cube's inspector look like:

My code for timer attach to cube:

 using UnityEngine;
 using System.Collections;
  
 public class Timer: MonoBehaviour 
 {
   private float startTime;
   private float elapsedTime;
  
   void Awake(){
  startTime = 0;      
   }
  
   void Update () {
  
   if (startTime > 0)
     {
        elapsedTime = Time.time - startTime;
     }
   }
 
   void OnTriggerEnter(){
     startTime = Time.time;
   }
  
 void OnTriggerExit(){
     startTime = 0;
   }
  
   void OnGUI(){
  GUI.Label(new Rect(300, 100, 100, 20), (elapsedTime.ToString()));
   }
 }

If I go near to cube - timer doesn't count time and display 0.