I'd like to use a fridge and some astronomical devices when the car is off. Unfortunately both of my 12V outlet provide no continuous power and some tricks(current of OBD is small, find no relay to short) are not working for me. However, I find a way best suited to me.
There is a main positive input line on top of the acceleration pedal fuse box, and the fastening bolt just has enough extra length allowing me to insert a spade lug and a M6 nut. Also, I use a mini fuse holder for the positive and use 14AWG silicone wire. The negative is from nearby bolt that fix a plastic to aluminium post.
I run the cloth-tape-wrapped wires alongside the center storage and through a hole under the bar into the under side of the center console, and install a heavy duty 12V socket onto another pre-drilled hole.
I've tested with an 180 watts portable electronic load that it will run 13.2A continuously without any difficulty.
One disadvantage for this approach is that the opening is upward prone to dropping metal objects and short the circuit. I must be careful to place the build-in cover when not in use. I originally planned to install the socket alongside the existing 12V outlet but find out it take too much to remove stuffs that I'm not comfort with.