I have a multiple select like this one could be:

 <select multiple id="brand">
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="opel">Opel</option>
  <option value="audi">Audi</option>
</select> 

And I'm looking for a way to end up like:

 <select multiple id="brand">
  <option value="volvo" selected="selected">Volvo</option>
  <option value="saab" selected="selected">Saab</option>
  <option value="opel" selected="selected">Opel</option>
  <option value="audi" selected="selected">Audi</option>
</select>

I can't depend on external libraries and I can't make the .each function to work properly. Besides I'm getting the original dinamically set with many fields, and this will be set to this only in a particular case, but i can't figure out why this isn't working for me. The closest approach i found is:

$("#brand").each(function(){$(this).attr("selected","selected");});

Can you please point me out where I'm going wrong? Thanks