I have it running through into the driver side door into the door cup holder area. Not sure where to mount it and I'm too afraid of cutting into my brand new car to do it myself lol. I honestly don't even notice it's there, so I haven't gotten around to looking into getting it done.
For the interior we used a 2" twin mattress topper I got for $30 at Walmart. I also got two storage bins that fit nicely under the platform. These stored pretty much everything but our cooler and clothing. A couple blankets and our pillows. Both of us had a restful sleep and our dog did well...
Here's some images of the platform. I used a sheet of plywood, 2x4s, outdoor carpet, wood screws, and a hinge for the head section. With the head section up the bed can be laid down with the seats in a normal driving position. With the head section (8 inches) down the seats must be pushed...
Went up to Pine, AZ for a test run with our dog and with the sleeping platform I made. We really enjoyed this setup and will make tweaks as we figure out our preferences.
I built a sleeping platform for myself, my gf, and our dog. haven't actually camped in it yet (that's about 2 weeks away), but it seems like it will do the trick. It stacks in the back so the back seats can be utilized.