second tow bar electrics? HELP

[highlander]

Im in the process of wiring in a second tow bar electrics for charging my caravan’s battery and running my fridge when im driving.
I need to connect one of the wires to a wire coming of the alternator that reads an output of zero volts when the ignition is on but not running!
When the engine is then running the output should be 12ish volts +.
Can any one tell me where to connect this wire as im having difficulty locating it with my volt meter???