Seems to be a common issue across all kinds of cars. What you mentioned is brought up a lot with people saying it fixed it and others not having success.
I am wondering if the issue is the headlight wiring coming directly from the headlight switch and getting too close to the radio speaker wires and/or the radio ground and power wires as well as the illumination on the radio as it’s part of the lighting circuit on the car.
Just me but so would see if isolating the headlight power with relays would fix the problem. Mount the relays up by the head lights and have the carrot wiring trigger the relays. Maybe do a search on this and see if it solves the problem.
Something like this so you do not cut up your factory harness:
