My power antenna needs to be replaced on the 745. In the interests of not buying a new power antenna, general exterior aesthetics, and my personal disdain for power antennas, I want to install a window-mount amplified antenna at the back of the car.

I intend to use the power antenna wiring for the 12v power this antenna will need, but I'm not sure which wire to splice into. In the photo you can see the car wiring harness on the left, and the power antenna harness on the right. The wires go as follows:

2x red/green wires on car harness connect to 1x red/green wire on antenna harness.
1x green/gray wire on car harness connect to 1x red wire on antenna harness.
2x black wires on car harness connect to 1x black wire on antenna harness

I assume that the black is the ground, and that the gray/green and red/greens are 12v constant power and the other is 12v power when the radio is turned on...but I have no idea which one is which.

Basically, I want to wire the antenna's 12v source into the wire that will turn the antenna on when the radio comes on. Of course I want to make sure I keep both FM and AM reception.

I really need to buy a multimeter.

Any and all help is appreciated.