How much do you think I can negiotate this car down?

Its a 2007 ford mustang premium for $12500 with a navigation system. It has a little more then 85000 miles with a v8 engine and is manual. If I pay in full all at once would the dealer be willing to take less?

Check kbb.com and nadaguides.com to find the value of this car. I agree with Jim that a V8 manual Mustang has a good chance of having been driven hard and possibly not maintained properly. Get a mechanic to inspect it before you buy.

Probably not going to get it much lower, Have it checked out mechanically, before you make any offer. A V8 manual Mustang may have had 85,000 miles of abuse.
If it is good, try $10.000 cash. But if it really is good, $12,500 is not unreasonable

Say you will grape his family if he does not give you 100% off. That or you can hire a girl to show some cleavage that might be able to more easily haggle him down while he is not fully focused.

Not much less.

Tell him all you have is 9500, and be prepared to walk away. Leave your number in case he changes his mind.