I am not sure where you get the 12V supply, does anyone knows if it comes from the 12kWH battery, or if it comes from the small one 12V battery in the trunk?
Also will the 12V in the trunk be charged with the same connection as the 12kWh battery or only from the Engines alternator? I Think(but not know!) that it must be electrically charged from the "wall", if not it must be drained if you have a driving cycle that allowes the gas Engine to not start for weeks.
This will have an impact on where you connect the 12V, you can get it from the starter on the Engine that i am almost 100% sure are always connected to the 12V in the trunk, but then I want to be sure how this battery will be charged, this could in odd usage cycles drain the 12V battery to the level that the Engine will not start.
Also, a normal car has a teoretical voltage of 14.4V when the engine is running, what do we have?, if you drain the small 12V battery i the trunk with 20A, it will not take long before you are close to 12V, this will have a huge impact on the light you will get out from Halogen lamps, LEDs are not that sensitive as the forward voltage is 3.3V to the LED.