protected function extract_make( $wp_obj ) {

			if ( ! $this->is_request_for_vehicle( $wp_obj ) ) {
				return '';
			}

			// if this is a search, the make might be in its own query variable.
			// $wp_obj is not a WP_Query and does not have a get() method.
			if ( isset( $wp_obj->query_vars['make'] ) && '' !== $wp_obj->query_vars['make'] ) {
				return $wp_obj->query_vars['make'];
			}

			// if this is a request for a single vehicle, parse the make out of the slug.
			if ( isset( $wp_obj->query_vars[ INVP::POST_TYPE ] )
				&& '' !== $wp_obj->query_vars[ INVP::POST_TYPE ] ) {
				$slug_pieces = explode( '-', $wp_obj->query_vars[ INVP::POST_TYPE ] );
				if ( 2 <= count( $slug_pieces )
					// is the first piece a number of no more than 4 digits?
					// We are looking for a slug like 2000-acura-integra.
					&& 4 >= strlen( $slug_pieces[0] )
					&& is_numeric( $slug_pieces[0] )
				) {
					return $slug_pieces[1];
				}
			}

			return '';
		}