I needed to charge my gps from my bike, it has a usb cable too.
I went to autozone and got a 12v cigarette lighter splitter, like this one here
, but make sure that it has positive and negative wires labeled (neg will be black, pos could be black with a white stripe). There are much better options, but this was cheapest at the time. I took the positive lead and tapped into the (IIRC) license plate light wire, with an inline fuse similar to this one.
Take the negative and ground it to the frame somewhere convenient.
For stuff bigger than a gps or phone, you will want to make a separate accessory circuit connected to the battery with a switch somewhere in there. (so you don't drain the battery accidentally)
you DO NOT want to start splicing same-colored wires from entirely different sources into each other. Computer and automotive wires will most definitely not be labeled similarly on purpose. You're really making it harder than it needs to be.
...and I just realized that the thread has already been resolved.