I'm not 100% sure on the trunk button. I'm 100% sure that the body control module is what commands the trunk lid solenoid to open. And 100% sure that shorting any wires that lead to the BCM(or adding +12 volts) can be a mistake. Learned that while trying to fix a power lock issue, using old reasoning. The reason things are the way they are now, is so the wires can be thinner, and cars are much more unlikely to catch fire, also more then one switch can be on the same two wires. Old cars were pretty much HARD WIRED with 12 volts and 10 to 30 amp fuse. The fuse was safety, and some people choose to override, resulting in fires. Newer cars you can have three switches on the same two wires, needing less voltage, less current and less copper. You press a switch and BCM gets 4.7ohms it knows your asking to lock the doors, 2.5ohms, your asking to unlock the doors(I made up the values, but this is how the newer stuff is working). You can actually add cruise to a cobalt by adding the switch to steering wheel and programming the BCM(tech2 needed) to tell the car it has it. It's just looking for values to know what buttons are been pressed(there is one wire you have to run from the BCM to clock spring also). GM has a tech bulletin discouraging factory personal from doing it.
