Descubrí cómo tener múltiples marcadores con ventanas de información, pero no se cierran cuando haces clic en otro marcador, creo que es porque estoy creando una nueva ventana de información para cada marcador, cualquier ayuda sería apreciada.

    <script type="text/javascript">


var map ;
  function initialize() {
    var latlng = new google.maps.LatLng(53.063165, -3.205390);
    var myOptions = {
      zoom: 6,
      center: latlng,
      mapTypeId: google.maps.MapTypeId.SATELLITE,
      zoomControl: false,
      mapTypeControl: false,
      mapTypeControlOptions: {
          style: google.maps.MapTypeControlStyle.DROPDOWN_MENU,
          position: google.maps.ControlPosition.LEFT_CENTER
      },
panControl: false,
streetViewControl: false,
      streetViewControlOptions: {
          position: google.maps.ControlPosition.LEFT_CENTER
      }
};
    map = new google.maps.Map(document.getElementById("map_canvas"), myOptions);

  var marker = add_marker(56.747923,-3.717155,"Ben Vrackie","<b>Ben Vrackie</b><br><br>Perthshire classic! some sweet hidden gems on this loop.<br> surrounded by amazing countryside, Lochs, hills this<br>has it all... Red Grade."); // pass in as Latitude, then Longitude
    marker.setMap(map);

   var marker = add_marker(57.556366,-5.409222,"Torridon Forest","<b>Torridon Forest</b><br><br>Technical and breathtaking.... <br>Black/red grade."); // pass in as Latitude, then Longitude
    marker.setMap(map)
}


  function add_marker(lat,lng,title,box_html) {

    var infowindow = new google.maps.InfoWindow({
        content: box_html
    });

    var marker = new google.maps.Marker({
          position: new google.maps.LatLng(lat,lng),
          map: map,
          title: title
    });


    google.maps.event.addListener(marker, 'click', function() {
        infowindow.open(map,marker);
});
return marker;
  }



 </script>