A company by the name of MaxPlay have released a Demo of one of their devices, which lets a Wii U owner use his Wii U in the comfort of his own car, here is the video followed by a quote from the website:
Functions & Features:
Play Wii U games via Wii U gamepad controller in your car! Not only for playing the Wii U games in front of your TV, you are also able to play the Wii U games via Wii U gamepad controller by using the Maxplay adapter inside the vehicle, finally something entertaining in a car, it should be great news for passengers
The Maxplay adapter can be used for charging the Wii U gamepad controller.
The Maxplay adapter also works as USB voltage output charger for charging most of the USB devices that get their power through USB (such as mobile phone, tablet, ipad etc)
Comes with Micro USB cable.
The device will be hand for those of you wanting to set your Wii U up in your car, my guess is though, that some of you could probably make a custom car adapter.