<?php

class Contractor_model
{

    function get_ids_from_tax($taxonomy, $taxarray, $operator="IN")
    {
        global $wp_query;
        $args = array(
                'post_type' => 'contractors',
                'posts_per_page' => -1,
                'tax_query' => array(
                    array(
                        'taxonomy' => $taxonomy,
                        'terms' => $taxarray,
                        'field' => 'id'
                    )
                )
            );
        $query = new WP_Query($args);

        return $query;
    wp_reset_postdata();
   }

}

$contractor = new Contractor_model();
print_r($contrator->get_ids_from_tax('service_type', '$_GET['service_type']'));

//$_GET['service_type'] returns the following:
//Array ( [0] => 24 )
//id #24 for the custom taxonomy service_type does exist, and it has 2 contractors attached to it

?>