How do I make it so that when my player enters an area, a disabled script attached to my player becomes active?

Questions & Answers
1

Title asks all.

2

You must create a trigger volume and use the OnTriggerEnter event. For the trigger volume: create an empty object (menu Game Object/Create Empty) and add a box collider (menu Component/Physics/Box Collider), then adjust its dimensions, position, rotation etc. and set the Is Trigger check box. It’s a good idea to identify the trigger by name or tag, so that only the right trigger does the desired effect - let’s call it “Area1”, for instance.
The OnTriggerEnter event is sent to both, the player and the trigger - the easiest solution in your case is to add the OnTriggerEnter function to the script you want to enable (this event occurs even in disabled scripts):

function OnTriggerEnter(other: Collider){
  if (other.name == "Area1"){
    this.enabled = true; // enable the script
  }
}

NOTE: For this to work, the player must be a CharacterController or Rigidbody, or the trigger must have a kinematic Rigidbody.