Discount Hawaii Car Rental is inexpensive and consistently gets great reviews. Because you do not pay until the day you pick up the car, we did this first and then played with Priceline's Name Your Own Price until we got the bid excepted at the rate we wanted.
We are also getting married in April. I suggest you book your rental car as soon as possible, because Maui only has a limited fleet of vehicles and, depending when you are going since Easter and Spring Break are this month, prices will start to sky-rocket as they sell out.
When I booked our wedding, the first thing we booked was a rental car reservation.