How to draw GL Lines over GameObjects/Images?

I am drawing lines between GameObjects to show their connection (showing a graph) using GL Lines and it always worked fine until I created a background Image using a Sprite. GameObjects are always hiding the Lines and because of the Background Image I can't see the lines anymore. Someone else had the reverse problem here: http://answers.unity3d.com/questions/434827/gllines-over-game-objects.html and I tried to use this shader but it doesn't work for me.

The code I have under OnPostRender() looks like this:

 Shader shader = Shader.Find("Lines/Colored Blended");
 Material lineMat = new Material(shader);
 GL.PushMatrix();
 lineMat.SetPass(0);
 GL.LoadOrtho();
 for (int i = 0; i < drawingGraph.edges.Count; i++)
 {
     Vector3 start = startObj.transform.position;
     Vector3 end = endObj.transform.position;
     GL.Begin(GL.LINES);
         GL.Vertex3(start.x / Screen.width, start.y / Screen.height, start.z);
         GL.Vertex3(end.x / Screen.width, end.y / Screen.height, end.z);
     GL.End();
 }
 GL.PopMatrix();

The shader I'm using is this from the other post:

 Shader "Lines/Colored Blended" {
     SubShader{
         Tags { "RenderType" = "Overdraw" }
         Pass {
             ZWrite Off
             ZTest Always
             Cull Off
             Fog { Mode Off }
             BindChannels {
                 Bind "vertex", vertex Bind "color", color 
             }
         }
     } 
 }

And that's how it looks without the background GameObject: And here with the background:

Can you guys tell me what I'm doing wrong? Thanks!