Power output on your vehicle's head unit is not designed to charge large mobile devices (e.g. TRX7 and other larger tablets). Doing so may compromise your experience with the TRX7 and, in some cases, result in loss of your account data.
You have two primary options to charge your TRX7 in your vehicle:
1. Connect the TRX7 to the Vehicle Power Adapter (VPA; provided with your TRX7) via the mini-USB cable.