Why is it that when I set my XRay shader as Camera's replacementShader , all gameobjects disappear ??

Basically for my game , I want to provide a XRay scanning feature , so that player can find gameobjects hidden inside other objects. So I made another camera render its view on a texture which is applied on a plane fixed onto main camera 'like a HUD'. Problem is when I set the camera's replacementShader to my XRay shader , all objects disappear . I guess problem lies in setting the replacement shader as XRay shader works as expected when attached on gameobjects. I attach shader as follows. Shader is attached on a material for time being so that I can modify it's uniform values.

  xrayCam.SetReplacementShader(mat.shader, null);

Shader code is as follows

 Shader "Custom/XRayShader" {
         Properties {
         _Color("_Color", Color) = (0,1,0,1)
         _Inside("_Inside", Range(0,1) ) = 0.2
         _Rim("_Rim", Range(0,2) ) = 1
     }
     SubShader {
         Tags { "Queue" = "Transparent" }
         LOD 80
         
     Pass {
         Name "Darken"
         Cull off
         Zwrite off
         Blend dstcolor zero
                 
         CGPROGRAM
         
         #pragma vertex vert_surf
         #pragma fragment frag_surf
         #pragma fragmentoption ARB_precision_hint_fastest
         //#pragma multi_compile_fwdbase
 
         #include "HLSLSupport.cginc"
         #include "UnityCG.cginc"
 
 
         struct v2f_surf {
               half4 pos     : SV_POSITION;
               fixed4 finalColor : COLOR;
         };
         
         uniform half4 _Color;
         uniform half _Rim;
         uniform half _Inside;
 
         v2f_surf vert_surf (appdata_base v) {
         v2f_surf o;
             
             o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
             half3 uv = mul( (float3x3)UNITY_MATRIX_IT_MV, v.normal );
             uv = normalize(uv);
             o.finalColor = lerp(half4(1,1,1,1),_Color,saturate(max(1- pow (uv.z,_Rim),_Inside)));
             return o;
         }
         
         fixed4 frag_surf (v2f_surf IN) : COLOR {
 
             return IN.finalColor;
         }
  
     ENDCG
     }
     
     Pass {
         Name "Lighten"
         Cull off
         Zwrite off
         Blend oneminusdstcolor one
                 
         CGPROGRAM
         
         #pragma vertex vert_surf
         #pragma fragment frag_surf
         #pragma fragmentoption ARB_precision_hint_fastest
         //#pragma multi_compile_fwdbase
 
         #include "HLSLSupport.cginc"
         #include "UnityCG.cginc"
 
 
         struct v2f_surf {
               half4 pos     : SV_POSITION;
               fixed4 finalColor : COLOR;
         };
         
         uniform half4 _Color;
         uniform half _Rim;
         uniform half _Inside;
 
         v2f_surf vert_surf (appdata_base v) {
         v2f_surf o;
             
             o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
             half3 uv = mul( (float3x3)UNITY_MATRIX_IT_MV, v.normal );
             uv = normalize(uv);
             o.finalColor = lerp(half4(0,0,0,0),_Color,saturate(max(1- pow (uv.z,_Rim),_Inside)));
             return o;
         }
         
         fixed4 frag_surf (v2f_surf IN) : COLOR {
 
             return IN.finalColor;
         }
  
     ENDCG
     }
 }
   //FallBack "Diffuse"
 }

Any Ideas as to what might be the issue ?? Any other better approach to achieve the the feature is also welcome .

Thanks... Nishant