I installed a second accessory socket today. The Powerlet low clearance one. I chose this instead of the BMW accessory socket because the BMW one wires up to the one in the cockpit. This means that it is controlled by the canbus and shares the same current draw limitation that the stock socket does, which could be problematic if you wear electrics in the cold weather. Additionally, the canbus shuts off the connectivity of the OEM sockets, and I wanted a socket wired straight to the battery (albeit fused) so that I can attach a trickle charger to it for the winter months.
You need to locate the hole carefully so that there is enough space for the part of the socket that protrudes into the area where it is mounted. Take great care in choosing this spot. I used a 5/8 drill bit and then hogged it out with a knife little by little until I had the hole large enough for the threaded portion to fit through.
As you run the wiring back to the battery, you'll note that with a little lifting and flexing that you can route the wires under existing bundles.
With the seat height forward adjuster pulled out of the way, it is easy to see that there is a "tunnel" of sorts that wires use to leave the under seat area. You want to run your wires through there.
As the wires run through that tunnel, they rest in a sort of plastic tray. The tray has a notch in it, and that is where I suggest your wires exit the tray and head down toward the battery. You may find that pushing a wire through the tunnel along the tray is a little easier if you pull down slightly on the tray, from the side just above the battery.
I also added a splash-proof inline fuse holder to the leads, for safety's sake. You can find them at any auto parts store.
Take extra care when attaching the positive lead. I chose to attach mine to the jump starting pin. I noticed that the area it is affixed to is insulated with rubber. It appeared to me that you need to be careful here or you will inadvertently ground the positive to the frame. :yikes So, use an appropriately sized eyelet and be careful with the angle and routing.