Awesome install. I literally just did the same thing with a Kindle Fire using tetherGPS. My HU is in the glovebox like yours, but I'm looking to mount it somewhere else in the near future as my glovebox handle has been broken for years and it takes two hands to open the glove box.
What are you using for profiles in tasker to save battery? Have you found it possible to control the on/off state of the kindle when charging? I lose about 6% battery overnight with the following settings, but I imagine that someone has come up with a better battery management profile.
When my kindle sees power it turns on wifi and connects to my phones network, GPS, turns the screen on and volume up and opens maps.
When my kindle loses power it turns the screen blackout to 7 seconds, turns off wifi, GPS and quits tetherGPS.