Hi Steve, sorry to not see your post sooner, but I have the same set up with cables feeding front and rear cams, so thought you may be interested in my set up.
Firstly, I have a 4 output usb adapter plugged into the central power socket. Then I have 5 metre flat usb leads, which come out of the front of the storage box (with it closed) and drop down under seats, then go off to feed cams front and rear - pushed under door trim etc.
The leads remain plugged into the adapter, until I visit car parks where they get plugged into an external battery, which powers them for at least 24 hours.
I can add direct links to cables, 4 output adapter and external battery if you require them.