Well it will depend on where you go. Generally they won't go past the total corporate discount especially if you have to order one.
Try your best though in negotiations, they're more likely to move on accessories than on the price of the car past corporate.
Also it will depend on which company you work for / organisation you're in as to the level of discount.
My advice would be to go to more than one place to see what their 'corporate' price is - I found that it can change a bit - they try to keep the cream from the top.
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