From what I remember the last time I rented in Miami, the rental car companies located 'on the premises' are just the counters located in the airport. You still have to take a shuttle to a remote lot to pick up your car. At least this is how it was for sure with National and if I remember correctly for the others as well.
You may want to check mousesavers.com for rental codes. Most of the rental discount codes listed there work for the whole state of Florida.