This confuses me, as well. Perhaps there's an unusual "route" for the power somewhere in that particular vehicle? Maybe there's some kind of a wiring fault that causes the tuner to somehow be supplying electricity to the ignition line?

Are you using the extension cable to the tuner module? Try removing it temporarily and plugging the tuner directly to the plug on the empeg.

Ooo, hey, you know, here is something that would cause the behavior you describe: If you are using the power antenna wire on the tuner, make sure it isn't connected to the ignition wire in any way. It should only be connected to the wire that powers the radio antenna, nothing else. Are you sure that's connected right?