Put your complaint in writing and keep a copy of the letter for your records. Send copies of any relevant documentation (such as receipts or contract terms and conditions), but hold on to the originals.
As before, send them to someone senior enough to have the power to help you - the manager of whoever you view as responsible for the problem is a good starting point.
It's in their interest to ensure their staff are doing the job properly, and they'll want to cover their own back in case the complaint goes further.
Take a measured approach. Writing 'I WANT MY MONEY BACK' in capitals doesn't win the argument, and a string of exclamation marks won't prove that you are in the right.
Keep to the facts and be brief. There are very few complaints that can't be summed up in a side or two of A4. Also, a mix of 'carrot and stick' can work wonders.
For example, a line like 'I hope this matter can be resolved quickly so I can continue to use your garage in the future' is more effective than 'I will never take my car to your garage again'. Companies will help more readily if, by resolving the complaint, they keep your business.
Setting deadlines is a good idea. A week gives someone plenty of time to investigate a problem and suggest a way forward.
E-mails are quicker than snail-mail, but the good old-fashioned letter still has its place. If the dealer principal gets 40 e-mails a day, yours will soon be off the bottom of the screen. However, in the days of the paperless office, a letter will stare accusingly from the in-tray until it has been dealt with.