Ignition sense is a function of the control head. In a dash mount config, the rear DB-15 (pin 5 ignition) is routed to the control head through the radio chassis.
When you remote mount a Spectra, there is no wire in the control cable to bring ignition up from the rear DB-15 to the head. In a A4, A5, or A7 control head, the control cable has a pigtail ending with a DB-15 female at the head end. This is where you connect a single ignition wire (pin 5 again) that routes to the control head. If you have a 9000 (A9) head, you'll have a green and orange wire just sticking out below the head connector. As mentioned, hook both to a switched source.
To use the radio rear DB-15 for ignition sense in a remote config, you'll have to hijack onto a spare or un-needed wire in the control cable to bring it forward to the control head. Wouldn't do this without a schematic to make sure the wire isn't connected to something else that will blow the radio's mind. Big hack and a lotta work for a minor advantage in the install department.