It's possibly a faulty headunit and was probably working normal when it was inspected since u said its intermittent.. and most likely its going to work normal when you take it to the dealership.. ..
Your local subaru can order it .. Its called a Trunk Release Actuator. . Somewhere around 40 to 50 bucks.. online from 30 to 40 bucks... you'll need a panel popper tool and a few sockets n ratchets to change it out...
Sounds like the reverse trigger is causing something with the headunit.... Did the car have an aftermarket stereo at some point , then went back to stock when they traded in the car ?
Thank you .. I figured it out . Took a while to find but Two of the wires that connect to the trunk actuator were broken. Solder them back together and all is fine now..
For some odd reason I cannot find a fuse for the trunk light .. I looked in the manual and it has nothing for trunk light fuse .. I've searched and came up with nothing for 5th gen ..