If you are not very tall, then you won't see the dirt and not need to clean it. (Kinda like the top of the refrigerator.)
Really though. I bought a 3 step folding stool that I keep in the garage. It's light and I can move it around the car as I clean/wax it. I wonder what people who have the real SUVs that are even higher do??
I open the doors one by one and stand on the sills to wash the roof. Of course I close the doors when I rinse it down. Unfortunately there's nothing to stand on at the back bumper. At 6'1" I can just barely reach back to the antenna from the rear doors. I can get the top of the rear spoiler and a few inches forward from the ground.
For washing I have a telescoping pole with some microfiber "strips" at the end. It looks like a mop. I don't recall where I got it since I've had it for a couple of years. I still have to stand on a ladder/stepstool, but the pole helps so I don't have to move the ladder around too much. I can pretty much get to the entire roof with only moving the ladder once or twice. I imagine if you're taller, you might be able to stand and use it.
Target sells a telescoping cloth bristle mop type instrument that plugs into your hose. I use this with a stepladder for the windshield and the roof. Works great. DIY's can prolly rig something with a wooden stick and a microfiber towel.