How can I display a different icon for markers that are suitable for different categories?

I am trying to create a Google map that allows the user to check the category and display these specific locations. I have this job:

http://thedenvillehub.com/test-hs/adv/scripts/test.html

What I want to do is add another icon for each category, so that it is red, one blue, etc. I found a couple of examples on the Internet, but nothing works for me. Here is my code for reference:

var markers = new Array();
var locations = [
    ['Boonton Town', '402-9410', 'Dial-a-Ride', 40.902, -74.407, 1 ],
    ['Boonton Township', '331-3336', 'Dial-a-Ride', 40.933, -74.425, 2 ],
    ['Butler Borough', '835-8885', 'Dial-a-Ride', 40.999497, -74.346326, 3 ]
    //other locations and categories
];

var map = new google.maps.Map(document.getElementById('map'), {
    zoom: 10,
    center: new google.maps.LatLng(40.7967667, -74.4815438),
    mapTypeId: google.maps.MapTypeId.ROADMAP
});

var infowindow = new google.maps.InfoWindow();
var marker, i;

for (i = 0; i < locations.length; i++) {  
  marker = new google.maps.Marker({
      position: new google.maps.LatLng(locations[i][3], locations[i][4]),
      map: map,
      icon: 'http://labs.google.com/ridefinder/images/mm_20_red.png'
  });

  markers.push(marker);
  google.maps.event.addListener(marker, 'click', (function(marker, i) {
      return function() {
          infowindow.setContent(locations[i][0] +
              "<br />"+locations[i][2]+"<br />"+locations[i][1]);
          infowindow.open(map, marker);
      }
  })(marker, i));
}

// shows all markers of a particular category,
// and ensures the checkbox is checked

function show(category) {
    for (var i=0; i<locations.length; i++) {
        if (locations[i][2] == category) {
            markers[i].setVisible(true);
        }
    }
}

// hides all markers of a particular category,
// and ensures the checkbox is cleared

function hide(category) {
    for (var i=0; i<locations.length; i++) {
        if (locations[i][2] == category) {
            markers[i].setVisible(false);
        }
    }
}

// == show or hide the categories initially ==

hide("Dial-a-Ride");
hide("American Legion");
hide("Veterans of Foreign Wars");
hide("Nutrition");

$(".checkbox").click(function(){
    var cat = $(this).attr("value");

    // If checked
    if ($(this).is(":checked")) {
        show(cat);
    }
    else {
        hide(cat);
    }
});