I should mention that this is my first carputer effort. I'm not using a touchscreen, but a $4 mini-keypad I bought off eBay:
I put in the switch to the left of the 12 V adapter as an on/off switch for the trunk power line.
Although this one seems like a ridiculous hack, I've used an old water bottle as an adapter for the inverter:
Haven't had any problems so far - seems to work just fine.