jQuery toggle

complete_code.js

   $("input[name='ballot[city]']").click(function() {
     if ($("input[name='ballot[city]']:checked").val() == "Other") {
       $("#ballot_other").removeAttr("disabled");
     } else {
       $("#ballot_other").attr("disabled", "disabled");
     }
   });

handling_change.js

   $("input[name='ballot[city]']").click(function() {
     if ($("input[name='ballot[city]']:checked").val() == "Other") {
       // enable text box
     } else {
       // disable text box
     }
   });

HTML_source.html

  <div class="field">
    <input id="ballot_city_quezon_city" name="ballot[city]" type="radio" value="Quezon City" />
    <label for="ballot_city_quezon_city">Quezon City</label>
  </div>
  <div class="field">
    <input id="ballot_city_manila" name="ballot[city]" type="radio" value="Manila" />
    <label for="ballot_city_manila">Manila</label>
  </div>

...

  </div>
  <div class="field">
    <input id="ballot_city_other" name="ballot[city]" type="radio" value="Other" />
    <label for="ballot_city_other">Other, please specify</label><br/>
    <input disabled="disabled" id="ballot_other" name="ballot[other]" size="30" type="text" />
  </div>