volvo 740 how to implement trunk unlock

Hey. I have one last thing to set up on my Omega Rec-12 keyless entry system and it's the trunk unlock function it has. The problem is with the way the car works: Normally, you unlock all the doors and the trunk by using the key in the driver's side door. Everything unlocks. And the trunk has a button you push to open it so you normally don't even need the key to pop it open, unless all the doors are locked.

Well for the Rec-12 I had to install a driver's side door lock motor (got that all working, with junkyard parts). So that's taken care of, and the remote works. But the problem is how to set up the trunk popper feature of the Rec-12? I had this idea to use a solenoid (I have a heavy duty one I bought a long time ago I can use) to basically pop the button from inside (I can see how to do it mechanically) with a cable, but the problem is the trunk needs to be unlocked for it to pop open. I thought of using a dei 528T with the BLACK/ stripe wire to pick up a negative pulse from the unlocking system then do a 1-second tug on the solenoid. Or I guess I could just connect the trunk output wire straight to the solenoid and then I just have to remember that the doors have to be unlocked before the trunk will open with the remote.

All the lock motors are 2-wire reversing polarity on the car. Any ideas? It's a 1991 Volvo 740.