Small claims court. There will be a fee for filing, but it's usually nominal. Once she is served with the papers, she may take your claim more seriously. The usual arrangement is that you will accept the $200 plus the costs to file. As the case proceeds through the system, the keep paying fees.... which she will need to pay to make it go away (so there is an incentive for her to pay up early). If you have a contract, if you actually end up sitting in front of a judge, it's pretty cut and dried. You have a contract, she can't prove she paid, she owes.
Before you file the claim, start writing down everything you've done to collect, sort of like a diary. You don't necessarily need written proof at this stage. But... things like - "Called X today, left a message at 4:12pm. Asked to hear back by [date]." Then, a couple of days after that date, write in your diary that you hadn't heard back, and that you called again. Then send a non-registered letter asking for payment (Politely! You don't want to lose the case based on harassment) ... be kind, make a joke, tell her you understand her situation, but you really need the money. Show the judge that you are being more than reasonable, and nice person. Put a copy of the letter in your file. Then send another, similar letter, by registered mail. Put the receipt that you sent it in your file.
As the case gets closer to court, she will likely be more inclined to settle. Be firm... demand the full $200 plus your court fees to date. If she wants to negotiate, maybe take $25 off. Some people just need that little victory to feel like they've won - but don't knock more than that off.
In court, you will have a thick file, full of papers, with the dates you tried to contact her. The contents of the letters, the expenses you incurred sending registered letters. It will show you tried to be sympathetic and a nice person. And she will have vague memories, extenuating circumstances, and excuses.
Documentation wins 95% of the time in a case like this. In this kind of court, the judge does not have to find in your favour "beyond a shadow of a doubt". In civil cases, it is simply the balance of probabilities.
You'll probably find that she will settle in the lobby of the court house when she sees you, and your file folder. Feel free to pad the folder with some brochures about the car. It will make the folder look more imposing, and if the judge asks to see the file the padding is still relevant to the case. Few people want to go in front of a judge when they think they will lose.
Good Luck.
For the good of society, take the time to show her that people shouldn't behave this way.