I hope that I can connect with you! I'm having the same issue you had with your Volvo S40 trunk on my daughters 08 S40. The trunk seems locked, the motor latch makes noise when pressing the outside release, but the it won't open even from the inside release. Makes me believe that it's locked and will not receive any signals to unlock.

My only question regarding your excellent depiction of your situation is the connection of the wire to the 9V battery. You said you took the 2 wire ends and inserted them into the two holes on a 9V. I don't follow as the 9V has a positive male post and negative female post but no holes. Did you attach both wires to a single post on the battery?

Looking forward to venturing into the trunk!

Best,

Chris