how to read data on wire

That would definately complicate things. 

You would have to make sure you don't tap the - side of solenoids or relays, as they will read 12vdc when they are waiting for a (-).  So, basically, you would need to tap a wire, and then drive the car and test every single function of the car before making sure you have the right wire.  You 'may' be able to just look for thicker wires, as thicker wires are generally feed wires, but even that isn't fool proof.