True ignition for a battery relay

Im looking for the best way to wire a Stinger battery relay. Stinger says to wire to 'true ignition', but i'm unclear on true ingnition. I guess i'm looking for a wire to show + when the ignition is on(4) and on the start (5)position, BUT show nothing when the ignition is turned all the back to accessory(1) position. I would like it this way so my batterys can disconnect when playing the radio with the engine off. Any suggegtions???? Its for a 2000 Lincoln Navigator.Alpine phantom Face CDA-7998

Alpine XMRadio,Alp. 12-disc ch

Kicker SX700.1, 2 SX1250.1 amps

2 kicker SS56.2 Components

1 12" SOLO-X 2-ohm dvc.

2 56.2sqFt boxes of hushmat

2 batt and H.O alt.....

you got th e basic idea,  i beliece it is blue/light green,  but double check with DMM.md