Text Prompt Problem

I'm having a problem trying to get this object to be interactive in this environment I am building. When you approach a sign with this "wanderer_figure" I want a prompt to pop up within the GUI layer asking the player to press space to activate further text to be shown. Right now I have it coded as such:

 var showGUI : boolean = false; 
 var customGUI : GUISkin;
 
 function OnTriggerEnter (myTrigger : Collider) {
      if (myTrigger.gameObject.name == "Wanderer_Figure"){
         showGUI = true;
         }   
 }
 
 function OnTriggerExit (myTrigger : Collider) {
     if (myTrigger.gameObject.name == "Wanderer_Figure"){
         showGUI = false;
         } 
 }
 
 function OnGUI() {
 //activate custom skin settings/textures
     GUI.skin = customGUI;
     if(showGUI){
         GUI.Box (Rect (Screen.width/2-100, Screen.height/2+200,200,100), "Press SPACE to Interact");
     }
 
     if (Input.GetKeyDown ("space")){
         showGUI = false;
         GUI.TextField (Rect (Screen.width/2-100, Screen.height/2+200, 400,200), "TEST");
     }
 }

However, when I try and test this code, the second half of the GetKeyDown call doesn't work, even if I hold down the space key. I'm trying to see what the problem, and also am wondering how I might make this second set of text disappear after a certain period of time (or else disappear based on more user input).

I'm kind of a n00b at this, so any help would be greatly appreciated, and thorough explanation of any tips would would equally be extremely helpful!