Yes, I said he was a tad special at times, and I recall when I was young installing a CB radio on my bicycle handlebars and a 12V battery on the rack. I did not have a BMW :-)
So, how is it done..
1. He has an inverter in the boot - sadly some of the kit works off mains. That does not mean we will not eventually make some DC to DC modules and do it all low voltage but for now an inverter is the simplest way. He has run a control switch to the centre console...
2. He is using an apple airport express as the wifi. Yes, he could use a MiFi for all this but that does not provide either Ethernet or AirPlay. The airport is a good wifi (802.11n) and has a 3.5mm jack to connect to the sound system (which is another story involving amps, subs, jigsaws and MDF). It means he can simply select his car on his iPhone or iPad to play music with no wires involved. Steve Jobs would have been proud.
3. He is using a FireBrick FB2700 and a dongle from t-mobile. Yes, not one of our SIMs as he plans to totally abuse their "unlimited data" and knows I would charge him if he used our SIMs. As expected, the dongle "just works" even with factory default config. That gives him an external (dynamic) IPv4 address - which looks un-NATted even, which is nice, but still only one IP and still only IPv4. He has Ethernet from the FireBrick to allow for someone with an in-car laptop, etc.
4. He is then using a VPN tunnel from the FireBrick to the big FireBrick FB6402 in our office. This allows me to route a /28 IPv4 (not greedy) and a /64 IPv6 (which, of course, includes 1337 as part of the address). No NAT proper Internet connection...
The brick does DHCP on the IPv4 space and Router Announcements on the IPv6.
We have, of course, set reverse DNS on both, and whois data for "James's BMW". I have not gone as far as polling google latitude and setting DNS LOC records, but one day maybe...
He has chosen to leave the WiFi open, but we can lock things down if ever needed on the FireBrick (it is a firewall, after all). Basically, if the dongle he has is unlimited, why not have an open wifi on your car!
So, in short, if you are ever on the motorway (perhaps near Addlestone?) near a black BMW and see wifi KE55ARD BMW (not not actually his number plate, though he would like that) then feel free to use the free in-car IPv6 wifi.