Flipping normals with shader - zeroing out

I'm trying to create a properly lit back-face shader. My plan is to use this with cloth objects, so I can apply two materials, one for the front face, and one for the back. I've set it up so the texture displays correctly, I just have one problem - lighting. If I don't change anything, the lighting appears the same as the front face, obviously, which looks odd, so I'm trying to flip the normals on the second material, which should allow the lighting to be processed correctly. Now, I'll admit, I'm not exactly a professional at writing shaders, so I may well be missing something extremely obvious, but this is what I have at the moment:

 Shader "Inverted Diffuse" {
 Properties {
     _Color ("Main Color", Color) = (1,1,1,1)
     _MainTex ("Front (RGB)", 2D) = "white" {}
 }
 SubShader {
     Tags { "RenderType"="Opaque" }
     LOD 200
     Cull Front
         CGPROGRAM
         #pragma surface surf Lambert
 
 
         sampler2D _MainTex;
         fixed4 _Color;
 
         struct Input {
             float2 uv_MainTex;
         };
 
         void surf (Input IN, inout SurfaceOutput o) {
             o.Normal = -o.Normal;
             fixed4 c = tex2D(_MainTex, float2(1-IN.uv_MainTex.x,IN.uv_MainTex.y)) * _Color;
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         ENDCG
         }
 
 Fallback "VertexLit"
 }

My issue is that the normals seem to be zeroing out, rather than facing the other direction. I tested this by rendering the normals as colour, and without any modification, they displayed a dull purple, while when multiplied by -1, they displayed black. Can anyone help me to work out how to flip the normals? Obviously, I can't just do it on the mesh, as I need to render the front face, too.