Update: I stripped down the Standard.shader and added the normal extrusion code. The problem is when _Amount > 0 then the passes won’t execute correctly.
_Amount = 0 
_Amount = 0.01 
The code is as follows:
Shader "Custom/StandardBlowUp"
{
Properties
{
_Amount("Extrusion Amount", Range(0,1)) = 0.0
_Color("Color", Color) = (1,1,1,1)
_MainTex("Albedo", 2D) = "white" {}
//_Cutoff("Alpha Cutoff", Range(0.0, 1.0)) = 0.5
//_Glossiness("Smoothness", Range(0.0, 1.0)) = 0.5
//_GlossMapScale("Smoothness Scale", Range(0.0, 1.0)) = 1.0
[Enum(Metallic Alpha,0,Albedo Alpha,1)] _SmoothnessTextureChannel("Smoothness texture channel", Float) = 0
[Gamma] _Metallic("Metallic", Range(0.0, 1.0)) = 0.0
//_MetallicGlossMap("Metallic", 2D) = "white" {}
[ToggleOff] _SpecularHighlights("Specular Highlights", Float) = 1.0
[ToggleOff] _GlossyReflections("Glossy Reflections", Float) = 1.0
[Enum(UV0,0,UV1,1)] _UVSec("UV Set for secondary textures", Float) = 0
// Blending state
[HideInInspector] _Mode("__mode", Float) = 0.0
[HideInInspector] _SrcBlend("__src", Float) = 1.0
[HideInInspector] _DstBlend("__dst", Float) = 0.0
[HideInInspector] _ZWrite("__zw", Float) = 1.0
}
CGINCLUDE
#define UNITY_SETUP_BRDF_INPUT MetallicSetup
ENDCG
SubShader
{
Tags{ "RenderType" = "Opaque" "PerformanceChecks" = "False" }
LOD 300
// ------------------------------------------------------------------
// Base forward pass (directional light, emission, lightmaps, ...)
CGPROGRAM
#pragma surface surf Standard vertex:vert
struct Input {
float2 uv_MainTex;
};
float _Amount;
void vert(inout appdata_full v) {
v.vertex.xyz += v.normal *_Amount;
}
sampler2D _MainTex;
void surf(Input IN, inout SurfaceOutputStandard o) {
fixed4 maintex = tex2D(_MainTex, IN.uv_MainTex);
o.Albedo = maintex.rgb;
o.Occlusion = maintex.g;
}
ENDCG
Pass
{
Name "FORWARD"
Tags{ "LightMode" = "ForwardBase" }
Blend[_SrcBlend][_DstBlend]
ZWrite[_ZWrite]
CGPROGRAM
#pragma target 3.0
// -------------------------------------
#pragma shader_feature _NORMALMAP
#pragma shader_feature _ _ALPHATEST_ON _ALPHABLEND_ON _ALPHAPREMULTIPLY_ON
#pragma shader_feature _EMISSION
#pragma shader_feature _METALLICGLOSSMAP
#pragma shader_feature ___ _DETAIL_MULX2
#pragma shader_feature _ _SMOOTHNESS_TEXTURE_ALBEDO_CHANNEL_A
#pragma shader_feature _ _SPECULARHIGHLIGHTS_OFF
#pragma shader_feature _ _GLOSSYREFLECTIONS_OFF
#pragma shader_feature _PARALLAXMAP
#pragma multi_compile_fwdbase
#pragma multi_compile_fog
#pragma multi_compile_instancing
// Uncomment the following line to enable dithering LOD crossfade. Note: there are more in the file to uncomment for other passes.
//#pragma multi_compile _ LOD_FADE_CROSSFADE
#pragma vertex vertBase
#pragma fragment fragBase
#pragma vertex:vert
#include "UnityStandardCoreForward.cginc"
ENDCG
}
// ------------------------------------------------------------------
// Additive forward pass (one light per pass)
Pass
{
Name "FORWARD_DELTA"
Tags{ "LightMode" = "ForwardAdd" }
Blend[_SrcBlend] One
Fog{ Color(0,0,0,0) } // in additive pass fog should be black
ZWrite Off
ZTest LEqual
CGPROGRAM
#pragma target 3.0
// -------------------------------------
#pragma shader_feature _NORMALMAP
#pragma shader_feature _ _ALPHATEST_ON _ALPHABLEND_ON _ALPHAPREMULTIPLY_ON
#pragma shader_feature _METALLICGLOSSMAP
#pragma shader_feature _ _SMOOTHNESS_TEXTURE_ALBEDO_CHANNEL_A
#pragma shader_feature _ _SPECULARHIGHLIGHTS_OFF
#pragma shader_feature ___ _DETAIL_MULX2
#pragma shader_feature _PARALLAXMAP
#pragma multi_compile_fwdadd_fullshadows
#pragma multi_compile_fog
// Uncomment the following line to enable dithering LOD crossfade. Note: there are more in the file to uncomment for other passes.
//#pragma multi_compile _ LOD_FADE_CROSSFADE
#pragma vertex vertAdd
#pragma fragment fragAdd
#pragma vertex:vert
#include "UnityStandardCoreForward.cginc"
ENDCG
}
// ------------------------------------------------------------------
// Normal Extrusion
}
FallBack "VertexLit"
//CustomEditor "StandardShaderGUI"
}
I have this shader to extrude normals, which can be found in the surface shader examples titled: “Normal Extrusion with Vertex Modifier”. 
Shader "Example/Normal Extrusion" {
Properties {
_MainTex ("Texture", 2D) = "white" {}
_Amount ("Extrusion Amount", Range(-1,1)) = 0.5
}
SubShader {
Tags { "RenderType" = "Opaque" }
CGPROGRAM
#pragma surface surf Lambert vertex:vert
struct Input {
float2 uv_MainTex;
};
float _Amount;
void vert (inout appdata_full v) {
v.vertex.xyz += v.normal * _Amount;
}
sampler2D _MainTex;
void surf (Input IN, inout SurfaceOutput o) {
o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
}
ENDCG
}
Fallback "Diffuse"
}
How do I add this to the Standard.shader?