{let attribute=$node.data_map.sidebar_boxes}

  {section show=$attribute.content.relation_list}
    {section var=Relations loop=$attribute.content.relation_list sequence=array( bglight, bgdark )}
      {section show=$Relations.item.in_trash|not()}
          {let object=fetch( content, object, hash( object_id, $Relations.item.contentobject_id ) )}<br />

            {section show=$object.main_node_id|null|not}

            {$object.current.data_map.header.data_text}
            {$object.current.data_map.content.data_text}

            <div class="image">{attribute_view_gui attribute=$object.current.data_map.media_library_object.content.contentobject_attributes.2}</div>

          {/let}


      {/section}
    {/section}
  {section-else}
    {'There are no related objects.'|i18n( 'design/standard/content/datatype' )}
  {/section}

{/let}

The sidebar_box is an object of type relations object.

Extract content objects from relations object in an  EZ Publish template

object

eZ Publish