Dont know if you still need help, open blocklayered.php.
Find line around line 1729:
$selected_filters[$attribute_name] = array($this->filterVar($url_parameters[0]), $this->filterVar($url_parameters[1]));
Add after this line another else if:
else if ($attribute_name == 'merk') {
$merk = str_replace("_", " ", $url_parameters[0]);
$merk2 = str_replace("_", "-", $url_parameters[0]);
$manufacturers = Db::getInstance()->executeS('select id_manufacturer from ps_manufacturer where name = "'.$merk.'"');
if(!empty($manufacturers)) {
$selected_filters['manufacturer'] = array($manufacturers[0]['id_manufacturer']);
} else {
$manufacturers = Db::getInstance()->executeS('select id_manufacturer from ps_manufacturer where name = "'.$merk2.'"');
if(!empty($manufacturers)) {
$selected_filters['manufacturer'] = array($manufacturers[0]['id_manufacturer']);
}
}
}
NOTE: replace "merk" in my code with "merken" for your code to work.
Now it should work!