As there for a new car you should try and use them as sweeteners on the deal. Once the sales manager has gone green and purple after you've took nearly all their profit out of the car then start on extra's and services.
I got most of their profit margin (from the profit margin figures I was given on these by a friend the dealer made 2% £320 across the car & extras) and then insisted on them including luxury mats and a devil exhaust too as the deal clincher

PS in the end I was sat with the sales manager, the dealer principal and a calculator for around 2 hours until the deal was done

I ended up getting the mats for free and paying £100 towards the exhaust.
Tell them you'll pay 50% and they pay the rest or you won't buy and see where you end up.
If you've already signed up for the car then best you'll do is probably 10% off. That said play hard as they usually have 30-40% in accessories. (Spot who's friend are all in the motor trade

) discounts to account customers (ie. other garages) are often 25-30% on parts.
Most of the profit in dealerships is in aftersales (parts, accessories and servicing) as theres very little in the cars (I've had dealers make as little as £50 on deals with me before now on cars).
Dave