Views Grouped Filters turns Radiobuttons into Select?
I  came up with writing a simple hook:

 function  MY_MODULE_form_views_exposed_form_alter(&amp;$form, &amp;$form_state, $form_id)
  {
    if ($form['#id'] == 'MYFORMID') {
      $form['field_MY_VALUE_FIELD']['#theme'] = 'select_as_checkboxes';
    }
  }