Hi,

I do not know anything about these newer cars but I can try to think with you on this conundrum.

If I understand your problem correctly it appears that the trunk only unlocks by using an electrical switch attached to a lever up in the drivers area. Much like a fuel door on some cars.

You say there is an option to bypass this process located in the trunk, probably near the latch, that you access when the trunk is open.

It surprises me or maybe it doesn’t!
Having a trunk that locks automatically without being told too, by the central door lock relays that locks up the whole car is odd.
The 1984 240 trunk has to have a key only but the car has electric door locks. So it has a totally separate key.
On todays cars there are two keys. Maybe the key you have is only what is called the valet key?
It keeps the valet out of the trunk and the console or glove boxes.

The only suggestion at this point is to get power back on in the car. This can be done at several points in a car since you access to everything else.
The most common place would be the starter connections as the battery runs power there and to the alternator systems first. A distribution point under the hood should start up there unless they put the distribution box back there at the battery?
I have seen BMW, in Junkyards, do that a run a massive cable with a 200 amp circuit breaker there!
That, I thought was crazy. Yes it protect the wire, but the amps up to that, can do a lot of damage before it trips.
With that thinking maybe it’s why it was already in the JY.
Good candidate of a car if you need a supply of battery cable wire.

Volvo can also get its power from the rest of the car there through a body circuit connector.
This can be where the fuse panel comes into play.
So with that said applying power to either of these two locations will then energize the relays to unlock things.

Ticket is, do you have the battery out or another one?
Just a couple of jumper wires and you can get back in business.

Phil