x2 wiring it into switched power, not constant power...those little USB adapters DO have a parasitic draw, don't ask me how I know.
OP, where are you located? I'm quite well versed in electrical and could help you install it if you're local.
And x2 wiring in a cigarette lighter plug instead of a dedicated USB plug - cigarette lighter plug gives you far more options, and you can also get a highspeed USB adapter for them vs the typically lower amperage outputs that the small motorcycle USB plugs provide. A phablet like an iPhone 6 will take FOREVER to charge on a 1000ma USB plug, but on a 2000-3000ma USB plug, dramatic difference.