How to negotiate on a new car and how low can you price the car?

I'm trying to buy a brand new 2014 car and these are my options. Ford Mustang I've seen it priced as low as 17,500
Chevy Mustang 18,300. Volkswagen Passat 16,700. How do I go about negotiating and what how much do you think I can get any of these cars for priced at the lowest.

NEVER negotiate price from full retail down. Always start at the invoice price and negotiate up. Make them an offer startying at the invoice price minus all the rebates and incentives offered. Make the dealer justify charging you more than the invoice price. Don't try to get them to discount the price down from full MSRP.