Several taxonomy query (like filter)

You would have to add the tax_query parameters depending on if the value is there in the GET parameters.

So it would be something like this,

$args = array(
           'post_type' => 'post',
           'tax_query' => array( 'relation' => 'AND', ) );
$car = $_GET["car"];
if(!empty($car)){
       array_push( 
                  $args['tax_query'],
                  array(
                          'taxonomy' => 'pa_my_taxonomy',
                          'field'    => 'name',
                          'terms'    => $car,
                       )
                 );
}