When I started trouble shooting mine I was using a long piece of 12 gauge wire directly connected to the ground at the battery. When I tied it to the amps I ran it under the car and up through a hole in the body then tie wrapped it out of the way.
I was just making guess though because mine was inconstant and when ever I went to work on it it was gone.
For your light it seems like a ground problem caused by it actually would show up when the light comes on since the circuit is closed. So it might not be the light is the problem rather it might be that something else has a bad ground and the light when turned on is fixing it.
See if disconnecting the ground of the light makes the buzz come back when it the trunk is open. that would indicate to me that something else is the problem.
Electrical issues always drive me nuts.
Mine only happened if the car was running so if I had the ignition set to On or Acc there was no buzz. I started thinking it was some component running the engine was going bad.
I stop trying to figure it out because it got to summer and since the buzz only happened when it was cold it was gone until the next winter. Then I unfortunately wrecked the car.
Wish I could tell you I solved it and how.