[moltuae]

It's something I've been meaning to do for a long time but haven't got around to it yet.

I did fit one to one of my previous 8s (mainly for a 12v cool box I used to carry on long journeys). I just took a permanent feed from the battery area, via an inline fuse. I used something like this (cut the plug off and wired it in) and attached it inside the boot using Velcro. Worked ok, but I think I'd probably go for something a bit more OEM-looking next time.

I've been contemplating designing a circuit for this that would automatically turn the power off to the power outlet when the engine wasn't running (to save looking for a switched live and therefore simplify installation). I thought something might already exist but I couldn't find anything last time I searched. It should be quite simple to do using a couple of comparators (to detect the upper/lower voltage thresholds) and an R/S flip-flop. When the battery voltage is above the preset level (ie engine running, battery charging), the flip-flop is set, turning on a relay, applying power to the outlet. When the voltage drops below the lower threshold (ie battery no longer charging), the flip-flop is reset, turning the relay off. Alternatively it could all be done with a microcontroller, making the thresholds (and any time delays, etc) programmable.