You can get better car rental prices through Priceline. Do not do the bid thing where you have to pre-pay just price out the car through the site. You can book it without having to pre-pay for it.
Trying to avoid booking your own car and flight may end up costing you a bit more money.
I also think trying to get 12 nights, flights and a car for 12 nights at $1500 or under might take a miracle.