My son is in the market for a new car. He wants to put a cash down payment to keep his payments low. Question - and please don't be a car salesman : is it better for us to obtain the most favorable terms if we tell the dealership ahead of negotiating the final price that we will be putting a cash down payment ? Or, should we hold off on that until after we agree on the final purchase price ? Can the dealership force us to reveal ahead of time how we will be paying for the car ? Can we refuse to answer this question and simply say :"you will get your money for the car one way or another" ? Thanks

First, the distinction between new and used car - I assume you mean actual 'new' car, not new for your son. Financing the two is worlds apart. A new-new car has tons of options for very low or even 0% financing, depending on the manufacturer's deal of the moment. You should see signs all over the showrooms as well as ads on TV. You don't have to get financing at the dealer, of course. But these days it's worth it.
You don't have to tell the dealer a single thing when negotiating, but negotiating is partly all about how you will pay, so it's an art. Yes, say you plan to put down a hefty down payment, have good credit, and a good job. The number one thing to do is NEVER sign a contract the day you walk in. Exchange info with the salesperson and tell him you are going to X other places too before you decide. Yes, it will be annoying to be called, so give your email only. You may find that the price goes down within a day or two, if it didn't while you walked around.

Go online to get not only dealer bluebook and dealer 'cost' (sort of an iffy number, depending on what it includes), but also sites that compare the average sales price for specific make and models around your zip. Prices do vary around the country.

Now is the time to buy a new car. End of fiscal year for the dealer, not the car year, since 2013's have been out for months, but this is a good time to get a new 2012. February is the absolute final sale days for cars, but by then there's not much good inventory left.