The key if you're not paying cash is to say this is what I want to finance and at your choice interest rate.
Do not tell them what payment you want as they will fiddle with the numbers to get you your payment at a higher rate.
Do not tell them what you want for your car. Just say: "I want to finance this amount for this many years at this interest rate.
They will say "We can't do that"
Then you leave.
Then they can suddently do that.
I just went to 3 different dealerships with the same numbers. I got my choice of car, at my interest rate, and financed the amount I wanted.
It's easy, just leave.