OK so it's update time.
I got a powered 4 port hub, and plugged that into the dc/dc convertor, and then fed the Tactrix and the Nexus from it. and you know what, finally it works!!!!!!!!!!!!!!!!! The battery doesn't drain 30% overnight. Now when the car is parked during the day at work, or overnight at home, it loses 3% at worst, but most times either nothing or 1%. So a big thanks to Andy_Mac for that tip.
Tonight I put in the FSL21 unit I got from Japan via Import Monster, the Beringer 202 I got from ebay, and also ran a USB extension lead to the centre console ( from the 4 port hub ) so either charge a phone in emergency, but more to be able to upload music to the Nexus without having to pull the dash to bits.
So it's 99% there, I just need to figure out why google maps doesn't work offline most of the time.