Ground (aka Earth) is the return path for the 12V dc system. It is the chassis of the vehicle. One conductor takes the 12V positive to the trunk (aka boot) latch solenoid. The negative conductor from the solenoid is connected to vehicle ground. If that connection is open circuit, the solenoid will not operate, even if the positive side is energised by operating the latch release ( I assume you have checked that that happens?)