Target Box not bounding targets

Hey guys, I wrote this script to draw a box around targets on my screen:

 var spaceObj : Transform;
 var targetTexture : Texture;
 var cam : Camera;
 private var screenPos : Vector3;
 
 function applyBox () {
 
     screenPos = cam.camera.WorldToScreenPoint (spaceObj.position);
     
     if (spaceObj.renderer.isVisible) {
         GUI.DrawTexture(Rect(screenPos.x, screenPos.y, 50, 50), targetTexture, ScaleMode.ScaleToFit, true, 0);
     }
 }
 
 function OnGUI () {
     applyBox();
 }

It gives me no errors but it simply doesn't work correctly: the box is not in the right position (the x axis looks ok, the y one is messed up: it goes above or below the target depending on my up&down movements). I think it's a camera-related problem.. I'm currenly using a third-person camera with smooth follow script attached..

How can I fix it? Thanks in advance ;)