Some people do have problems with AJAX part, when loading a make, the models do not appear.
A webhoster from Holland has solved this. We don't say it's always working, but it's a very nice work around.
Try to make the next changes:

Open the next file:  /administrator/components/com_rdautos/views/application/tmpl/form.php

Replace the following code:

function createModels(index)
{
    var obj = document.getElementById('modelid');
    eval(ajax[index].response);    // Executing the response from Ajax as Javascript code   
}

With this code:

function createModels(index)
{
  var obj = document.getElementById('modelid');
  var str = ajax[index].response;
  eval(str.substring(44)); // Executing the response from Ajax as Javascript code
}