I've picked up a DC-DC charger that uses a sense wire to tell it when the motor is running, so it will kick in and start charging.

I'm hoping someone can tell me where I could connect this sense wire? I don't want to hook it up to any accessory wires as I don't want the charger to start up when I'm parked and listening to the radio for example, so it needs to be only when the engine is running.

The charger and the battery it charges are both inside the vehicle, so if there's somewhere inside that'd be great.

Thanks.