Integrate QMT funcktions into existing Form

Got it working now, without hte plugin. Thanks to

https://www.ads-software.com/support/topic/search-multiple-taxonomies-witch-checkboxes

Just use the following code in your functions.php

//explode checkboxes array when doing a search
	$_GET = yclads_implode_checkboxes_taxonomies($_GET);

function yclads_implode_checkboxes_taxonomies($request_var) {

	$args = array('your tax here');

	foreach($request_var as $arg_key=>$arg_value) {
		unset($value);
		if (!in_array($arg_key,$args)) continue;
		$value= $request_var[$arg_key];
		if (!$value) continue;
		if (!is_array($value)) continue;
		$new_args[$arg_key]=implode(',',$value); //'OR' operator
	}

	$new_args = array_filter((array)$new_args);
	$output = wp_parse_args( $new_args, $request_var );

	//DEBUG
	$yclads_debug['checkboxes_args_before']=$request_var;
	$yclads_debug['checkboxes_args_after']=$output;

	return $output;
}