They don't take a huge amount of current; you could buy one of those in-car universal adaptors - just make sure it's a *regulated* one. These usually plug into the cigarette lighter socket and have a range of tips you can use - about $10-$15 for a decent one. If you're hiding it away you can do a neater permanent wiring, too.

One thing is that you don't want to use a native 12v ethernet client (ok, it's not a client, but an example is something like the apple airport) as you *need* some over voltage/reverse voltage protection too. Going through a regulator gives you this for free.

Hugo