I found it much easier to connect to the dome light cluster on the roof.
Dont remember which wire was connected to the accessory power, but it
is easy to check with the multimeter.
You will need separate earth
connection (none of 4 wires from the light cluster are connected to
earth),
but there is a convenient earth point above, next to the antenna.
This is 12DC, so i hooked one standard USB power supply for 12V and standard USB - mini USB cable.