What would be a true ignition source for relay/isolator if a 1997 Mustang?

[asilentfire]

I've been trying to find a true ignition source for my Install Bay IB500 500 amp relay/isolator in my 1997 Mustang coupe. I have a second battery and capacitor in the trunk that I want to separate from the battery under the hood when the engine is not running, this way if I leave my system running all night the starting battery under the hood will remain at full charge.

From what I understand, the isolator has the batteries disconnected by default, and in order to connect them I need to supply 12v to the ignition terminal to bridge the connection. And because I only want them connected when the car is running the remote turn on lead on my head unit will not work because that gets power (thus connecting the batteries) when in accessory mode with the engine off, defeating the purpose.

I heard of 3 true ignition sources:

1. Ignition wire
2. Hot wire
3. ACC wire on head unit (?)

I don't know anything about hot wiring cars so I don't know where ignition wire, or hot wire is. I know that under the driver side dash there are a bunch or wires but that's about it.

Sorry if I made this question too long, I just wanted to provide as much (mis)information as I could to try to help others in the future looking for an ignition source.

[asilentfire]

I'm ordering a Littlefuse add-a-circuit kit to tap into the fuse box in the engine compartment, but I don't know what fuse to use yet. I've heard of solutions involving using the fuse for the windshield wipers or fuel pump, but that involved a "buffer" relay to prevent fuses from burning out or something.. I'd rather just use the relay/isolator I have.

[asilentfire]

I could really use some help here, this seems to be one of the internets great unanswered questions, but a lot of people know the answer!

[fasfocus00]

DMM? wiring diagram? guess not. try power window or windshield wipers those usually only get power when the car started.

[subzero]

Id use a series of relays and switches and make my own.