Not as extensively tested as I normally do, but here you go
[rgss]#--------------------------------------------------------------------------
# * Auto Command
# A simple script that adds an auto command to the Party Command bar
# in the default battle system
# Created by Racheal of Dragonfly 02/22/07
# Editted 05/14/09 to include continuous auto
#--------------------------------------------------------------------------
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#--------------------------------------------------------------------------
# * Game Temp edit
#--------------------------------------------------------------------------
class Game_Temp
 alias old_initialize initialize
 #--------------------------------------------------------------------------
 # * Public Instance Variables
 #--------------------------------------------------------------------------
 attr_accessor :auto_battle          # flag for auto battle
 #--------------------------------------------------------------------------
 # * Object Initialization
 #--------------------------------------------------------------------------
 def initialize
  old_initialize
  @auto_battle = false
 end
end
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#--------------------------------------------------------------------------
# * Battle Spriteset edit
# * Change @auto.x and @auto.y to change the position of the auto icon
#--------------------------------------------------------------------------
class Spriteset_Battle
 alias old_initialize initialize
 alias old_update update
 alias old_dispose dispose
 #--------------------------------------------------------------------------
 # * Object Initialization
 #--------------------------------------------------------------------------
 def initialize
  @auto = Sprite.new
  @auto.bitmap = RPG::Cache.icon("auto")
  #Change the two lines below to change the position
  @auto.x = 5
  @auto.y = 5
  @auto.z = 400
  @auto.visible = $game_temp.auto_battle
  old_initialize
 end
 #--------------------------------------------------------------------------
 # * Dispose
 #--------------------------------------------------------------------------
 def dispose
  @auto.dispose
  old_dispose
 end
 #--------------------------------------------------------------------------
 # * Update
 #--------------------------------------------------------------------------
 def update
  @auto.visible = $game_temp.auto_battle
  old_update
 end
end
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#--------------------------------------------------------------------------
# * Party Command Window edit
#--------------------------------------------------------------------------
class Window_PartyCommand < Window_Selectable
 #--------------------------------------------------------------------------
 # * Object Initialization
 #--------------------------------------------------------------------------
 def initialize
  super(0, 0, 640, 64)
  self.contents = Bitmap.new(width - 32, height - 32)
  self.back_opacity = 160
  @commands = ["Fight", "Auto", "Escape"]
  @item_max = 3
  @column_max = 3
  draw_item(0, normal_color)
  draw_item(1, normal_color)
  draw_item(2, $game_temp.battle_can_escape ? normal_color : disabled_color)
  self.active = false
  self.visible = false
  self.index = 0
 end
 #--------------------------------------------------------------------------
 # * Draw Item
 #  index : item number
 #  color : text character color
 #--------------------------------------------------------------------------
 def draw_item(index, color)
  self.contents.font.color = color
  rect = Rect.new(80 + index * 160 + 4, 0, 128 - 10, 32)
  self.contents.fill_rect(rect, Color.new(0, 0, 0, 0))
  self.contents.draw_text(rect, @commands[index], 1)
 end
 #--------------------------------------------------------------------------
 # * Cursor Rectangle Update
 #--------------------------------------------------------------------------
 def update_cursor_rect
  self.cursor_rect.set(80 + index * 160, 0, 128, 32)
 end
end
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#--------------------------------------------------------------------------
# * Scene Battle edit
#--------------------------------------------------------------------------
class Scene_Battle
 alias old_end battle_end
 alias old_update update
 alias old_start2 start_phase2
 #--------------------------------------------------------------------------
 # * Battle Ends
 #   result : results (0:win 1:lose 2:escape)
 #--------------------------------------------------------------------------
 def battle_end(result)
  # Cancels auto after battle is complete
  $game_temp.auto_battle = false
  old_end(result)
 end
 #--------------------------------------------------------------------------
 # * Frame Update
 #--------------------------------------------------------------------------
 def update
  # Cancel auto with the cancel button
  if Input.trigger?(Input::B)
   $game_temp.auto_battle = false
  end
  old_update
 end
 #--------------------------------------------------------------------------
 # * Start Party Command Phase
 #--------------------------------------------------------------------------
 def start_phase2
  old_start2
  # Auto process if auto in effect
  if $game_temp.auto_battle
   update_phase2_auto
  end
 end
 #--------------------------------------------------------------------------
 # * Frame Update (party command phase)
 #--------------------------------------------------------------------------
 def update_phase2
  # If C button was pressed
  if Input.trigger?(Input::C)
   # Branch by party command window cursor position
   case @party_command_window.index
   when 0  # fight
    # Play decision SE
    $game_system.se_play($data_system.decision_se)
    # Start actor command phase
    start_phase3
   when 1  # auto
    # Play decision SE
    $game_system.se_play($data_system.decision_se)
    # Auto processing
    $game_temp.auto_battle = true
    update_phase2_auto
   when 2  # escape
    # If it's not possible to escape
    if $game_temp.battle_can_escape == false
     # Play buzzer SE
     $game_system.se_play($data_system.buzzer_se)
     return
    end
    # Play decision SE
    $game_system.se_play($data_system.decision_se)
    # Escape processing
    update_phase2_escape
   end
   return
  end
 end
 #--------------------------------------------------------------------------
 # * Frame Update (party command phase: auto)
 #--------------------------------------------------------------------------
 def update_phase2_auto
  # Clear all party member actions
  $game_party.clear_actions
  for actor in $game_party.actors
   if actor.inputable?
    # Set action
    actor.current_action.kind = 0
    actor.current_action.basic = 0
    actor.current_action.decide_random_target_for_actor
   end
  end
  # Start main phase
  start_phase4
 end
end
[/rgss]
Insert all of this into a new script below Scene_Battle. You'll need an icon called 'auto' in the icons folder.
Here's one I did up quickly for testing purposes. You can edit it's position in the Spriteset_Battle section of the script where marked. If for some reason you don't want an icon indicating whether auto is activated or not, simply remove the Spriteset_Battle section of the script completely.