As someone who had installed a home-made 3g hotspot in his car about 10 years ago using a Sierra Wireless 3g PC-CARD in a Phoebus 3G Wifi router, (and later replaced that with a much smaller Cradlepoint 3g-wifi router) these days I really don't see the need for car-as-wifi-hotspot anymore with LTE on almost every device, and personal hotspots offered from most cell phone carriers.
My iPhone has long since replaced the Cradlepoint since the jailbreaking days and an app called MyWi. Now the LTE hotspot is just included in my AT&T plan and works great.. and for Mac devices (Macbook Air, iPad), I use Bluetooth instead of wifi for internet sharing as it uses less bandwidth and doesn't broadcast an SSID.
So I just don't see "car-as-hotspot" such a big deal anymore.
p.s. For sale: Phoebus and Cradlepoint cellular wifi hotspots.