A suggestion: write up a letter addressed to the BBB, make a copy for the dealer, make a copy for a local news station that is known for accepting consumer complaints, and make a copy for DC Corporate. Take all four to your dealer, and very calmly & very politely state to them the problem you are having, mention your level of frustration without anger, ask them if your assessment of the situation is accurate, and tell them that you intend to send these letters because you feel you have been given no other option.
The letter itself should be relatively brief [nmt 3 paragraphs], it should clearly state the dealer name, the service failure, the steps you took to allow the dealer to make it right and the issue that remains unresolved. State facts, only briefly mention your level of frustration but don't dwell on it.
Do your homework and list each of the mailing addresses on the letter, so each party knows who else has a copy of the letter.
Based on what you've posted here, it sounds like the problem is happening at the dealer level; involving Corporate may get you bumped up the list, but some sort of penalty could get levied against the dealer... so even if you end up getting your car from them, I wouldn't risk getting service done there if the dealer holds a grudge.
Your dealer is in the best position to actually help you in the short term, and they should be given the one last chance to earn your business; whether they will strive to do that depends on whether they feel you have been acting fairly yourself.
Having the letters in hand shows them you mean business. Don't expect the dealer to move mountains or change the laws of space & time; and if you were nasty with them the last time you left the dealership, don't expect them to jump to help you. You are giving them a chance to make it right, but being calm and fair while doing so could make them want to make it right. Act like the loyal customer that you long to be, let them know you feel betrayed, and give them the chance to make it right. If the dealer wants to and can do something at that point, they will. If they don't, that's their problem. Don't waste another minute, ask for your deposit back, send those letters, and go somewhere else to place an order. It may take longer to get your car, but at least you can feel good knowing that the initial dealer didn't take any of your money.
At the new dealer, be sure to let them know what happened at the first place, act like the loyal customer that you long to be, and let them deal with you fairly... if the two dealers are close enough to be competing for business, you never know what could happen.
Let us know how it turns out.
If you feel wronged enough to make a legal case out of it, it is possible you could win, but by the time you did, the 300C may not be the coolest thing out there any more, and you may be too old to drive!
