I don't have a Fronty, but I have slept in the backseat of my X. I'm only 5'8", but sleeping in the backseat kinda sucks. It is OK for a few hours (4-5), but if you're looking for a good nights sleep, I wouldn't recommend it.
Don't do it! A few weeks ago i slept across the back bench during a SAR training mission in the boonies. I'm about 5'9" or 10" too, and my legs were at a 90-degree angle all night long, because I was too lazy to set up the tent i had with me in the bed or on the ground. I'm a side sleeper so I knew stretching out on a reclined seat would not work well for me either. I've learned my lesson.
Yea not sure how well the back seat, or any seat for that matter is going to work. If you are serious about sleeping well in your truck or the like, I'd recommend looking into a Roof Top Tent and mounting it over the bed as many have done. I feel they are worth the money and will be investing in one soon my self for either the FJ or the Frontier. There are lots of choices out there and lots of different prices ranges.