(trunk) wiring diagram/information?

[dominii]

I've built myself a kickass little addon for my '08 128. It's a Bluetooth-accessible LED array with a complementing Android app. So, from my phone, I can send any text I want out my back window.

However, right now, I'm powering it via a set of 4 AA batteries. I would like to find a wire in the trunk that can supply power to the board when the car is turned on. I need 5 or 12V and a very small amount of amps.

Anyone know if there's a wire in the trunk I can get access to for this? Or is there a wiring diagram for the car or trunk somewhere I can use for research? ... Or should I break out the multimeter.

Thanks!