What data type are you using for your FirstYear_new and LastYear_new fields in your database? Looks like they might be varchar, that could throw off your search results...
Also looks like you have your >= and <= mixed up.
seems like it should be:
$WADbSearch1->addComparison("IDModel_new","".((isset($_POST["model"]))?$_POST["model"]:"") ."","AND","=",1);
$WADbSearch1->addComparison("FirstYear_new","".((isset($_POST["year"]))?$_POST["year"]:"") ."","AND","<=",0);
$WADbSearch1->addComparison("LastYear_new","".((isset($_POST["year"]))?$_POST["year"]:"") ."","AND",">=",0);