There are four 100MM nuts securing the blower housing. Look at them as they may be rusted on the studs. If not too badly rusted spay with a quality penetrant. Not WD40. Let it sit and do its job. Hopefully the studs won't break when you try to remove them. If you remove the assembly here is a thread on replacing the blower motor. Heater Blower Unit Rebuild Tutorial - Mercedes-Benz Forum