Have you called the boss? He seems to be the one to approve sending the check. In a calm polite way say you've been waiting 3 months and have not yet received the check. Ask him why.
If that doesn't work, ask to speak to the highest authority that makes decisions about checks. Tell him, in a calm respectful way, that you want to know why you haven't been sent the check.
You seen to think "they" are purposely defrauding you. Of this is a large company doing business for years, it's more likely that there hasn't been effective communication up the line of responsibility. So you move up the line to be sure the person who signs the check knows what's happening.
The salesmen is only responsible for selling the car and making you happy to buy from him. Once you've bought the car, he moves on to the next possible buyer. He only makes money when he sells a car. Salesmen are just ordinary people. Some are conscientious and some not. It's possible they are saying what they want you to hear without following through to see if it happened. Salesmen have very little, maybe no control over what happens in the dealership. If the dealership is large, it's likely they don't even know who to call. The papers you received has a customer service number to call for help. Start with that person. Ask why the check hasn't been sent. If that person says it should've been sent, ask them what they can do to make this happen. Ask if you should talk to someone else. Customer service people are usually more knowledgeable than salesmen or their supervisor.
You do not need to go to the dealership. It's a waste of time and money. The check will likeky be sent from somewhere else.I doubt anyone in that building can write checks. You want to talk to the person who authorizes signing checks and then the office who actually signs and sends checks.
I suggest you decide how much time, effort, and money you're willing to spend to get $200. There have been times in my life that I settled for less or nothing at all so that I could have peace. Peace, being free of anger or irritation and having my life back was worth more than the money owed me.