Best place to get +12V ignition power from?

[Adam Smith 2]

I am installing a battery-isolating contactor under my hood, which will connect my starting battery to the deep-cycle battery in the rear of my truck, which is there to help with my high-amperage 12V air compressor. I only want the batteries connected when the alternator is running, so I'd like to know the easiest or best place to tap a source of +12V ignition power from.



I want 12 volts that is only available with the engine running (ignition switch pos 2) not accessory power like the stereo gets.

 

[Todd Z]

Under the fuse cover under the hood, In the middle I think there is a 15 or 20 fuse that is key on power.... i used that for the +12 key for my electric fan relay..



Todd Z