perhaps a wire is disconnected. SOmetimes the pin switch that activates the light can fail, leaving the light on all the time, draining the battery.
I would pull the light out, and follow the wires, and check the pin switch. I can make my interior lights go out easy by disconnecting a pin switch in the door, I usually do this when I am working ont he car and don't want to hear the key buzzing.
Check the trunk pin switch and light fixture for any disconnected wires. If you find a disconnected wire, you may want to test by pushing the pin switch and seeing if the light stays on or goes off.
Just an idea to help you solve the problem.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.