Yes, sorry. The Luxos U is dynamo powered. I use hub dynamos.

Of course no device will be able to charge beyond the maximum output capacity of the dynamo, but the advantage that this has is that the light and the charging circuitry come in one unit. There is no need to find another mount point for the charging device. No extra battery is required to ensure constant, even USB voltage output so device specific adjustments are not necessary.

Combined with a dynamo hub, this is more than enough to keep a Garmin bike GPS unit charged. I use a bicycle specific GPS, but I don't see why it wouldn't work with your car GPS. I guess it would depend on how much power your unit really uses. Bigger screens mean more juice is needed.

While I'm not very familiar with the auto GPS units or the mounting options, the Garmin bicycle units do mount to a bicycle quite well. Now, there are "Touring" models that cut out some of the racer/training fitness specific options, and a lot of the related cost.

Personally, I like to plot my routes before I ride, even with a bike GPS. I'd be even less comfortable relying on a car GPS to find routes that are bicycle friendly.