[Meir]

If you look at the diagram, you can see that the activation switch is not connected directly to the relays. It connects to the ECU, and the ECU is operating the relays.
The spoiler will extract at 75 Mph, and will retract at 55. So the ECU is managing the spoiler behavior.
You can also use the switch to manually activate the spoiler, but it will not work when the car is moving. My guess it is there for service purposes
Usually, in modern electric window system, there is some sort of a knock sensor that can detect if the motor stopt turning.
My guess is the ECU is responsible for that as well.
Since you are not going to install a Porsche ECU in your Subaru, this is my suggestion to you.
Take a regular after market electric window switch, and connect it directly to the spoiler motor. You will be able to extract an retract the spoilr yourself. No harm will happen if you will hold the switch for couple of seconds more.
If you won't to have it extracting and retracting all the way with one touch, you can use an after market roll-up window module, that will be activated from the switch.
I don't think you will be able to control it based on speed like the original way, without using a module that can calculate speed, and count the delay time, before extracting or retracting.

If I'm already here...
In my car, the spoiler will not work based on speed, but will work with the switch, so I assume I have a faulty connection between the relays to the ECU.
I think the relays are OK, because I can activated in the service mode (when the car is not moving) but maybe the relay has multiple circuits.
This way or the other, fixing the spoiler is on the bottom of my to do list, but if someone has an idea, you are welcome to shoot.

Silver, if you need more info ( diagrams, module recommendation etc. ) let me know.