ref: 2010 Chevy Express 3500 with 6.0L

Adding a high amp ignition relay [500 amp] to isolate auxiliary batteries from my original vehicle battery when key is off. Everything is installed but attaching to a wire that is only hot when the key is on or in accessories.

Wanting to find a switched/accessories hot wire [on when key on - off when key off] somewhere in the engine compartment that is easy to get to. Hoping to avoid running all the way to the fuse panel under the drivers seat or one in the engine compartment down on the inner fender [hard to get to and even harder to know which wire is which fuse slot]. I don't want to start piercing wires with a meter probe to test.

Can anyone provide a diagram, specific location, photo of such, and wire color? Thanks in advance!