Example package.json and Gruntfile.js files demonstrating installation of @lydell's fork and branch of grunt-autorefixer with support for Sass' sourcemaps.

Gruntfile.js

module.exports = function(grunt) {
  grunt.initConfig({
    pkg: grunt.file.readJSON('package.json'),
    sass: {
      build: {
        files: {
          'style.css': 'style.scss'
        },
        options: {
          sourcemap: true,
          style: 'expanded'
        }
      }
    },
    autoprefixer: {
      options: {
        // note the addition of this option over original grunt-autoprefixer
        sourcemap: true
      },
      build: {
        files: {
          'style.css' : 'style.css'
        },
      },
    },
    watch: {
      sass_watch: {
        files: ['*.scss'],
        tasks: ['css_build']
      },
      css_watch: {
        files: ['*.css'],
        options: {
          livereload: true,
        },
      },
    },
  });

  grunt.registerTask('default', ['watch'] );
  grunt.registerTask('css_build',  ['sass:build', 'autoprefixer:build']);
  require("matchdep").filterDev("grunt-*").forEach(grunt.loadNpmTasks);
};

package.json

{
  "name": "my-project",
  "title": "My Project",
  "version": "0.0.1",
  "author": "My Name",
  "private": true,
  "devDependencies": {
    "grunt": "~0.4.1",
    "matchdep": "*",
    "grunt-contrib-sass": "*",
    "grunt-contrib-watch": "*",
    "grunt-autoprefixer": "git+https://github.com/lydell/grunt-autoprefixer.git#source-maps"
  }
}