The problem on Spy Hunter, is that there is no place to grab 20V (no flashers). There is 11.9VDC unregulated at the PS board (TP3).
WARNING -- THIS IS NOT TESTED...JUST SOME IDEAS
But I'd try using a transistor switch -- Rc, is the load (the post LEDs), Rb would be about 10K ohm. Vcc would be connected to the power supply TP3, and the base resistor connected to the GI. The other problem, is that the GI is VAC, so you'll need a simple 4 diode rectifier, and a cap to keep the base of the transistor in saturation. You could probably build this circuit for a couple of bucks (not including the post LEDs).