Is the landing light grounded through the airframe in the wing?
If so, I suspect that the return current through the airframe is creating the issue.
As a test, try isolating the light from the airframe and running a temporary ground wire back to your ground block. If that fixes it, you can either run a permanent wire and declare victory or spend some time understanding where the airframe ground current is creating an issue with your GTN650/audio wiring.
Hope that helps,
David