It has been a few years since we have flown into SNA. I seem to remember the rental cars are located in the parking garage area. Do we walk there or need to take a shuttle?
We booked to SNA from Chicago for our October trip partially because the rental cars are literally across the street!! SOOOOO much easier!!! If you look up a map of SNA, you'll see where each of the "brands" are located. Signage is really good too. Keep an eye on when you return the car ... we returned at 5:45am in February. We forgot to fill up with gas. Super hard to find a gas station around the airport open that early. If you are mindful of that fact (we never pre-purchase gas), then you won't get dinged like we did for not returning full!
There are some bargain companies that aren't in the terminal parking garage area and you have to take a shuttle. Thrifty, Ace, Economy, Fox, Nu, USave.