Lightmapped object in Surface-shader

Hi,

It looks like my lighting model is ignored when an object has a lightmap. Is there are way to control how lightmap is blended with other stuff when using Surface-shader?

What I mean is if you do this (taken from Unity examples):

  Shader "Example/Diffuse Texture" {
     Properties {
       _MainTex ("Texture", 2D) = "white" {}
     }
     SubShader {
       Tags { "RenderType" = "Opaque" }
       CGPROGRAM
       #pragma surface surf SimpleLambert
 
       half4 LightingSimpleLambert (SurfaceOutput s, half3 lightDir, half atten) {
           half NdotL = dot (s.Normal, lightDir);
           half4 c;
           c.rgb = s.Albedo * _LightColor0.rgb * (NdotL * atten * 2);
           c.a = s.Alpha;
           return c;
       }
 
       struct Input {
           float2 uv_MainTex;
       };
       sampler2D _MainTex;
       void surf (Input IN, inout SurfaceOutput o) {
           o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
       }
       ENDCG
     }
     Fallback "Diffuse"
   }

LightingSimpleLambert is completely ignored on lightmapped objects? How do I control the strength of lightmap in the surface shader?