I did have power issues until I got the voltage to 5.3V at the plug. USB devices are designed to change their charging rate to adapt to what's supplying it. If the voltage drops, it reduces its requested current until things stabilize, so that way it won't draw more current than the adapter is designed to supply. I've noticed that most USB charging devices won't take full current unless the voltage is slightly higher than a regular USB port (and so it knows it's on a dedicated charger supply). Mine won't take enough power to charge while running unless the voltage is above 5.25V (at the plug) during use.
I'm estimating at the plug voltages since I can't really measure that easily, and of course there are some losses in the cable.
The supply definitely turns on when the ignition is on, and off when it's off, but the Nexus gets really confused. It may be because the car pulses the voltage supply when I turn the key briefly before coming on. It's clearly confused when it gets pulsed. When I have the key in the accessory position, then start it, the car interrupts power briefly. That confuses it into thinking it needs to turn off when the car is running. I hate that. Once it starts turning off, even though it waits a few seconds, it seems to continue to turn off even if it gets power back.
I'm running Multirom, so I switched back to Timur's Rom this morning, but I remembered now I have an issue where the left channel is weak. It's clearly software since it doesn't do it in Autodroid. What a pain. I'm getting frustrated. I'll see if I get pausing and the other issues with Timur's rom though. Clearly there is a lot of R&D associated with making a Nexus into a stable head unit! I was thinking that Android would be more stable than windows (for a carputer), but I'm wondering now (flame suit on