Trunk won't open...

Not sure if it's the same as Gen2 but if it is there should be a motor latch that you hear and if you don't you'll want to inspect the switch that's activated when you push rear hatch button on outside. That system often goes bad in Gen2, but think they fixed it in Gen 3. Other thing to check is the physical latch for dirt and grime. Get it clean and re-greased.

 

You would first want to find out whether the issue is more with the motorized latch or more with the switch or the keyless entry system.

Using Techstream or another scan tool with bidirectional controls, you can select Main Body / Active Test / Operate back door lock motor, and if the latch makes a sound and opens when you do that, then from the body ECU to the latch is ok.

Viewing the live Data List, you can watch Back Door Open Handle SW while a buddy pushes the handle switch. If that shows ON while the buddy is pushing, then from the switch to the body ECU is ok.

Of course the motorized latch can be OK, and the switch can be OK, and if the smart key ECU doesn't like the fob you're carrying for some reason (or if your car doesn't have all-door smart key entry, and you haven't unlocked the doors), it's still going to say Nope.

The Data List item Back Door Open shows the lock status, whether opening will be permitted or prohibited.

If it is showing Prohibit when you think it should be Permit, then the problem is more with the locking status.