Multiple form inputs droppable
The first thing I would do with this code is wrap the entire code in an IIFE.  This will create your own private scope and not pollute the global namespace.  Also, since you are using jQuery, you can pass in the jQuery object as $ to ensure that it is correctly mapped.

(function( $ ) {
  //your code here
})( jQuery );


Next, you have each droppable in its own document.ready call.  That is unnecessary.  You only need one call. We will add that in at the end of the code.

Since most of your code is actually in the event handler and it is almost entirely the same for each instance, we can DRY out your code. Just pass in a string that contains the one difference between each function and cache some of the variables.

function dropHandler( item ) {
  $('.ui-selected.' + item).each( function() {
    var $curr = $('#' + item), val = $curr.val(), html = $(this).html();
    $curr.val( ( val === "" ) ? html : val + ", " + html;
  });
}


With this in place, we just need a to loop over all of your items.  So add each input name into an array and loop over it creating the droppables.

var fields = ['dest_city','dest_state','car']

function init() {
  var len = fields.length;
  for( var i = 0; i &lt; len; i++ ) {
    $('#' + fields[i] ).droppable({
      drop : function(event, ui) {
        dropHandler( this.id );
      }
    });
  }
}


Last just add a call to the init function on document.ready.

$(function(){
  init();
});


After these changes, here is the updated code:

(function( $ ){

  var fields = ['dest_city','dest_state','car'];

  function dropHandler( item ) {
    $('.ui-selected.' + item).each( function() {
      var $curr = $('#' + item), val = $curr.val(), html = $(this).html();
      $curr.val( ( val === "" ) ? html : val + ", " + html;
    });
  }

  function init() {
    var len = fields.length;
    for( var i = 0; i &lt; len; i++ ) {
      $('#' + fields[i] ).droppable({
        drop : function(event, ui) {
          dropHandler( this.id );
        }
      });
    }
  }

  $(function(){
    init();
  });

})( jQuery );


Let me know if you have any questions or comments.