Please don't take this the wrong way but no one can tell you how to do something that's not in your nature. Someone can tell you how to negotiate this every step of the way with who to call, when to call, what to say, etc. and when you do it, it will only come across as phony and game playing and not only hurt your cause but make you feel uncomfortable the entire time.
A transaction is nothing more than both sides reaching a mutually agreeable point and making trade. It will never get there if one side feels like the other is being disingenuous and trying to take advantage of them.
Those guys want to make a sale. I'm sure that they're thinking every day that they had this one in the bag and they don't want to see it go away. Dude, you gave them a full price offer. Think that comes along every day? Think again. If the dealership owner and manager are worth their salt they would be calling you every day.