MB Hotrod

I just drove over the mountain in my wifes ML 350. Got to the hotel and the driver door won't lock? Just happened to be near the local MB Dealer so I stopped by and asked and of course the would have to order the part if they could figure out what the issu is. You know how it works today.. plug it into a computer and maybe it will give us a code as to what part to replace. I would love to buy this part from the aftermarket and repair myself. any thoughts??