I use a slim-fitting case on my cellular iPad Air. I use a T-Mobile SIM card with 200MB of free data every 30 days. I also use TomTom's USA GPS app, which is incredibly good. When I take a trip of 300 miles, I pay $1.99 for 30 days of traffic updates through TomTom, which needs a data connection. I play music from the iPad through my Bluetooth stereo and, when the TomTom USA GPS app gives directions through the stereo speakers, the music pauses, then resumes after the direction is given. The stereo is the base model that was included in my Scion (Toyota) xB.
In order to mount the iPad Air to my dashboard directly behind my steering wheel, where there are no gauges, I put a couple of velcro circles onto my dashboard and onto the iPad Air's case. The iPad Air is also being supported by the car's steering column for great support. When I need to glance down at the iPad Air to see the GPS instructions, it is as quick and easy as glancing down at a speedometer.
I also use a Lightning cable to keep the iPad Air charged, and so that I can say, "Hey Siri... play U2, or Open TomTom, or Apple stock price, or send a message to, etc."
I am very satisfied with the way that this setup works. Here is what it looks like:
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When I move on to the next car, I can take the iPad with me.