How about a quarter wave vertical with one of those multi leg radials you see on e bay at the bottom of the quarter wave or make one yourself and raise the antenna up off the car a little bit with a heavy strap or piece of solid rod. Mount it at the rear, on top attaching it between the lift gate and body, it just might work. Make a NMO mount if possible or an "N" mount, put a piece of welding rod where the wire is usually soldered as your radiator, the four radials in the 4 holes in the "N" mount and solder your coax to the center and one of the 4 legs at the mount solder the coax shield. This will of course be just a one band antenna, I have seen in some publications a piece of metal attached to the main rod that go out and then is bent down to make it a duel band. You also want to keep the feed as close to 50 ohms as possible. That may involve bending the radials downward some to get the antenna itself as close to 50 ohms as you can.
Get some of the flexable LMR 240 for coax, it will have the least loss in a coax that is the right size to run into the car.
The radials will give you your ground. It may not be the best set up, but your vehicle was not made for mounting antennas on it.
Other than that, adapt a front fender mount. Last thought, make or buy a roof rack that connects to the rain rails (I hope it has them), put a piece of metal across the roof mount, as big as possible and mount your antenna in the middle of the piece of metal using an NMO mount. If you do that, you can run an antenna with lots of gain. I'd ground the metal to the vehicle using some wire, cad welded on the metal or screw the wire down with star washers and cover the connection with black permitex so water will not make the connection go bad in time.
Get down to the metal where the other end of the wire is hooked to the car, use star washers to cut into the metal, that should give you a little more ground.
Either will be fine for local work or hitting repeaters that are not too far away, and the idea of metal on a roof rack will get you a lot more distance.
I just think it would be fun to see if you could build a quarter wave that would work on the back, it's going to need the radials back there as there is so little metal for a decent ground.
Next time buy a 1990 Chevy Suburban, there is a vehicle you can put an antenna farm on the roof. hi hi
I know some of the ideas are a little crazy, but they would work, main thing is look for the best way to mount the antenna that will get you a good signal and not hurt the vehicle too badly.
Your going to have to think outside of the box on this one if you want a very good signal; it can be done without messing up your vehicle.
Good Luck, John