I have, use, like the go pro 3+. I've used a gorilla pod like this (
http://www.photonlight.com/accessories/joby-gorillapod-camera-tripod-original/?utm_source=googleshopping&utm_medium=cse&gdftrk=gdfV21383_a_7c1216_a_7c5204_a_7cGORILLAPOD_d_GREY_d_1&gclid=CNDDvdrU0MQCFZY2aQodTAIAlw ) to wrap it around the rear view mirror. You can flip the image in GoPro settings. Synch it to your phone so you cans what the camera sees. Have an auxiliary power cord to the lighter as the battery life is limited. IF you have a buddy who works at Apple, they had an employee discount of 45% off when I got mine. I would prefer the 4 as the design of the battery compartment is far superior than the previous designs. TONS of mount options for other applications too. This (
http://www.dx.com) is a great site for aftermarket fittings a fraction of the 'GoPro' brand prices. Often identical, sometimes superior. See 'smattree' batteries as an example. Here's some insights on battery options and their performance compared to GoPro's.
http://www.reddit.com/r/gopro/comments/2j4eq9/which_battery_to_buy_hero4_silver/
Yes, underwater options. Time lapse is a cool way to do the road trip vids. Good Luck, hope this helps!