Where to get 5V in the trunk?
 

I've picked up a Soundgate BMWPDAUX1 and have been using it in my 1998 318ti on eBay for fairly cheap (under $50). The kit came with one main unit and some additional components that are meant for the old 30-pin iPod cable but I am using some other gear for bluetooth connectivity.

https://i.imgur.com/3tiTYbI.png

Lots of components but still fairly straight forward. The only issue, if you can believe it, is that I'm running a long ass USB extension from the lighter outlet in the front to the back at to power the USB Bluetooth thing.

Does anyone know where I can grab power in the trunk to for this thing? I've got spare components and can trivially add resistors and/or another lighter usb adapter to get 5v out of 12v but I'm not sure what power I can actually use.