For anyone still interested, the deal is over. I didn't get the best rate, but did get 5.75%. I didn't get the dealer to drop the price any further, but did get them to give me different wheels, which are worth a bit of money.
After getting approved by my bank, I went to the dealership to haggle. I acted like I wanted the car, but was tempted by Ford's incredible incentives on new cars. I hemmed and hawed about it, then offered $17,500. The salesman made the obligatory trip to the sales manager's office (he probably just went to the bathroom), came back and said $18,900 was as low as they could go. I suggested $18,000. He again went to his sales manager (or to the bathroom).
He brought out "the sheet." This is a piece of paper with numbers all over it that shows what the dealership supposedly paid for the car. It's bull. The sheet showed they'd paid $21,500. I told him that, if they had paid that much, they paid about $4000 to $5000 too much, which they wouldn't do. I gave him my card and told him to call me if he wished.
When I got home, I went to Kelly Blue Book and Edmunds, and entered all of the options this car has (there's very few it doesn't have). Based upon those options, condition of the car and mileage, the car was definitely worth more than $18,900. I'm sure that they told the previous owner that they'd give him $21,500 in trade for his new vehicle, but they weren't able to reduce the price of the new vehicle. So they essentially paid somewhere around $17K for the Mustang, and got a bit less than full value on the new vehicle.
After looking at that, I realized that $18,900 was probably as low as they could go, or pretty close to it. Their profit was probably about 10-12%. They had already reduced the price from $22,900 to $21,900 to $18,900.
I also found out why I hadn't been able to find the deep red exterior/tan interior color combination I wanted anywhere else: out of the 160,000+ Mustangs made in 2007, there were only 312 five-speed GT's made in that color combination. I could look the rest of my life and not find one for sale.
I called my brother to confirm my thoughts, then called the salesman, and said I'd be stopping by with a check. As I waited for him to get paperwork, I looked at the car and confirmed I'd made the right decision.
I drove it for all of twenty minutes today from the dealership to the storage building. Now I have to wait until April or so to actually get to drive it.
It's not a cheap car, and it's not a practical car. But it's fast, handles fantastic, and it makes me feel good. I'm just wondering how many tickets I'll have by the end of the summer.