My wife's car has a USB jack for the radio, but you can also use it for power. I decided to make a power splitter for this USB jack that would only split the power.
I cut off an end from an old cable and removed the molded end. I cut all of the wires but the black and red ones. I soldered these wires to the outermost headers
on the circuit board and covered it all in epoxy putty. I had to make sure I was bonding it in the right direction so I didn't end up with the jacks facing down.