First, remember that people sometimes hold multiple reservations because of special rates, promotions, etc. At some point, they're going to let those reservations go. Also, there are no-shows every day.

Second, the advice above about your hotel is correct. Most good concierges can come up with a car. If you're at a resort with a car rental desk, which many of them have, you can call that location now and get on a list. They may even be able to confirm you.

Check back every day. Check multiple sources. The car rental companies themselves and sites like travelocity, orbitz priceline, etc.

Like air awards, plane tickets, hotel rooms - inventories change all the time. I'll bet by the time you leave for your trip, you'll have it confirmed.

Good luck.