If you're getting nowhere, it's time for a bit more stick and a little less carrot.
Send the managing director a letter, briefly explaining your dissatisfaction at the way your complaint has been handled.
Even if they pass your letter on to someone else, at least you know the big boss is watching. Many will be glad that you've shown them how things are going wrong at the sharp end of their business, and they will want to make amends.
Again, request a response within a reasonable period, and write to them again if this deadline passes without any more progress.
If your complaint is with a dealer, write to the manufacturer, too. Yes, Customer Services are the obvious port of call, but they're masters in the art of politely telling you to get lost. Send your complaint to the managing director, too.