Event.observe(window, 'load', init, false);

function init(){
Event.observe('searchText', 'keyup', greet, false);
}

function greet(){
var url = 'includes/model-numbers.php';
var pars = 'searchText='+escape($F('searchText'));
var target = 'searchResults';
var myAjax = new Ajax.Updater(target, url, {method: 'get', parameters: pars});
}