search multiple taxonomies (witch checkboxes)

Hi, I would like to have a search form where the user is able to check several taxonomies to search in (using the OR operator).
so my checkboxes elements have
name="yclad_category[]"
for example, which is an array.

So I guess I have to implode this array before the query args are parsed.

I made this :

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

function yclads_implode_checkboxes_taxonomies($request_var) {

	$args = array('yclad_action','yclad_category');

	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;
}

when I print my $request->query_vars (using the hook parse_request), I got this

Array ( [post_type] => yclad [yclad_category] => cars,casting [taxonomy] => yclad_category [term] => cars,casting )

So by there it seems to be OK.
But then the query do not work. No posts are returned…

Any idea of why ?

THanks !