stuck my voltmeter where it wasn't welcome

[tranceatlantic]

Im trying to find a source of 12v in my rear trunk to power a piggy back computer and some diagnostic stuff. I need something that only provides power when the ignition is on. Sounds like a simple enough task right? So I have my voltmeter handy I started poking around in the large plastic connectors (X connectors I think thay are called in the Porsche manual) and looking in my manual for a possible wire on the DME cable bus. So far all I have manged to find is a wire that provides power for about 5 mins then shuts off (secondary air supply possibly?). Then, last night, I managed to trigger something. Dont know what but I turned something on and it started humming in the engine compartment. I shut off the ignition and it stopped. This morning my car wouldn't start, like the battery was drained. So it makes me want to stop poking my voltrmeter in places its clearly not welcome. Has anyone got a idea on where I might find such a wire back there that I can splice into?