Look on the transformer (the big black brick thing usually with the wall prongs on it) and you should see ~110AC/??DC where ?? is the voltage it convertts to. Get one that has a brick with a 12v or 5v output. Usually they are 5v because that is an electronics standard somewhat.
I see no reason why you can't run a molex from the trunk to the glovebox. Just use 2 wires and tap into the 5v/12v and a ground so you don't need the entire molex saving you half the space.
And running a nonpowered HUB off a powered HUB will achieve nothing but extra ports. The limitation is not within the HUB, it is within USB standards. 250mA total per USB, so if you take 250 from the mobo and split it and split it then you need to add some voltage to keep it at 250, instead of continually splitting it. Then when you get a powered HUB, it will give 250 out for each of its ports. Split off of that, and you again have lower voltage.