I advocate sticking with the major companies such as Hertz, Avis, Thrifty, etc. They all have convertibles. If a Mustang is a must, I would call the local Phoenix location and ask what convertibles they stock. I always rent from Hertz, and they offer both Mustang and Chrysler convertibles, and, though I can make a request on a reservation, there is no guarantee that a particular brand will be available on the day of rental.
Ditto Bobbear. And, convertibles are usually expensive to rent, so I suggest trying to find a coupon somewhere. Those Entertainment coupon books usually have them, or if you're a member of a travel group such as AAA, frequent flyer, etc. you can usually find discounts.
Be prepared to pay a big premium for a convertible. Hertz and Budget usually have good inventories of Ford products.
I'm doing the same thing in May and did the research and selected Avis. I did call locally and they have Mustangs but as previously stated "won't" guarantee them. The price is expensive. I joined the Avis Preferred Program hoping that might get me some clout. If nothing else you can bypass the counter and go directly to selecting your vehicle. Hope that helps.
You may wish to try Hotwire starting within a month of your trip to see if you can get a better price on a convertible (not bidding, actual prices). We did this, both on a recent trip to San Francisco, then when we went to Hawaii in November. We had a reservation booked with one of the major companies several months in advance & then I kept checking prices. For our Hawaii trip, the Hotwire prices went down probably within a week of our departure date (significantly!). Our reservation was routed to Hertz in SF, Avis in Hawaii. Then, of course, we cancelled the other reservations.
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