Is there a way to access uv coordinates in a custom lighting model?

Im trying to do this in a BRDF shader I've written and basically what I want to is have a different ramp texture used based on the mask texture provided.

This is my code:

 Shader "Learning/BRDF_FULL_MASK"
 {
 
 Properties {
 
     _Color ("Main Color", Color) = (1,1,1,1)
     _SpecColor ("Specular Color", Color) = (1,1,1,1)
     _Shininess ("Specualr Power", Range(-1,2)) = 0.5
     
     _MainTex ("Main Texture", 2D) = "white" {}
     _BumpMap ("Normal Texture", 2D) = "white" {}
     _MaskTex ("Mask (RGBA)", 2D) = "white" {}
     
     _Ramp_RED ("Shading Ramp 1", 2D) = "gray" {}
     _Ramp_GREEN ("Shading Ramp 2", 2D) = "gray" {}
     _Ramp_BLUE ("Shading Ramp 3", 2D) = "gray" {}
 
 }
 
 SubShader {
 
     Tags {"RenderType" = "Opaque"}
 
     CGPROGRAM
     #pragma surface surf BRDF_FULL
     #pragma target 3.0
     
     float _Shininess;
     float4 _Color;
     sampler2D _MainTex, _MaskTex, _BumpMap, _Ramp_RED, _Ramp_GREEN, _Ramp_BLUE;
     
     struct Input {
         float2 uv_MainTex;
         float2 uv_BumpMap;
     };
     
     struct SurfaceOutputCustom {
         
         fixed3 Albedo;
 
         fixed3 Normal;
 
         fixed3 Emission;
 
         half Specular;
 
         fixed Gloss;
 
         fixed Alpha;
         
         fixed2 uv_MaskTex;
 
     };
     
     half4 LightingBRDF_FULL (SurfaceOutputCustom s, half3 lightDir, half3 viewDir, half3 atten)
     {
         float2 uv_MaskTex;
         
         //Get the dot product of the sirface normal and light direction
         half NdotL = dot (s.Normal, lightDir);
         half NdotE = dot (s.Normal, viewDir);
         
         //Normalize diffuse
         half diff = NdotL * 0.3 + 0.5;
         
         //BRDF
         float2 brdfUV = float2 (NdotE * 0.8, diff);
         float3 rBRDF = tex2D(_Ramp_RED, uv_MaskTex).rgb;
         float3 gBRDF = tex2D(_Ramp_GREEN, uv_MaskTex).rgb;
         float3 bBRDF = tex2D(_Ramp_BLUE, uv_MaskTex).rgb;
             
         //Specular
         //half3 h = normalize(lightDir + viewDir);
         //float _Shininess = max (0, dot(s.Normal, h));
         //float _SpecColor = pow (_Shininess, 48.0);
         
         float3 m = tex2D(_MaskTex, uv_MaskTex).rgb;
     
         //half4 c = tex2D (_MainTex, IN.uv_MainTex);
     
         float3 res = m;
         
         if (m.r >= .1) res = rBRDF.rgb;
     
         else if (m.g >= .1) res = gBRDF.rgb;
     
         else if (m.b >= .1) res = bBRDF.rgb;
     
         half4 c;
         //c.rgb = (s.Albedo * _LightColor0.rgb * BRDF + _LightColor0.rgb * _SpecColor) * (NdotL * atten * 2);
         c.rgb = (s.Albedo * _LightColor0.rgb * res) * (atten * 2);
         c.a = s.Alpha;
         return c;
     
     }
     
     void surf (Input IN, inout SurfaceOutputCustom o) 
     {
         //Handle textures
         //Handle Diffuse map
         fixed4 tex = tex2D(_MainTex, IN.uv_MainTex);
         //Handle Normal map
         float3 bump = UnpackNormal (tex2D(_BumpMap, IN.uv_BumpMap));
         
         //Apply textures
         //Apply Diffuse texture
         o.Albedo = tex.rgb * _Color.rgb;
         //Apply Normal map
         o.Normal = bump.rgb;
         
         //Specular
         o.Specular = _Shininess;
         o.Gloss = tex.a;
     
     }
     
     ENDCG
 
 }
 
 Fallback "Diffuse"
 
 }

and this is the result I get from that:

and the shader mask and how I want the ramps to be applied to the model:

I think my problem is that I can't get uv coordinates calculated inside the custom lighting model. Is this possible? If it isn't what is a way to make this shader work?

p.s. The model isn't mine, I got it here - www.steffenunger.com.