The Honda Element doesn't have a "trunk" per se. It's a SUV like car but I believe it has a normal car engine. It's a boxy design so it's an open trunk just like a SUV's. Currently, it's marketed with a pull down back/trunk door where you can add the option of a backing so if you were the outdoorsy kind, you can just park your car and sit on the back/trunk door like a bench with the addon back rest. I've seen plenty of people keep their large breeds at the back area of their SUV and wagons. So it's the same idea. I can't imagine having a small breed ride back there though. Except when it's parked and used as a recreation vehicle, I can see leaving the trunk open/down for the dog to go in/out up and down the ramp on its own to use as its own bedroom/tent.
With all that said, even if I had considered purchasing a Honda Element, I would probably not opt to pay a premium for the "pet features". I can build my own ramp for less money or improvise for free. And probably put a bed there I like better or just use a crate at the back for the same purpose. I definitely wouldn't want a waterbowl attached to the car. It can become quite messy or unsanitary.
If I were parked, I'd put the bowl outside on the floor or in the crate also. But then that's just me and I don't even have a license! LOL.