2003 Sport HSE buzzing noise from speaker when I turn headlights on - HELP!

Hello from Montreal!

I've always been an Audi guy, but my lifestyle requires a vehicle with more cargo space and towing capacity. I picked up a low-mileage 2003 Range Rover Sport HSE. Clean inside and out. Very happy with my purchase, and happy to join the community.

The only problem with it (at the moment) is a buzzing noise coming from the rear passenger side door ONLY when I turn ON the headlights. Also the speakers were cutting in and out but now they seem to be fine.

The research I've done online has led me to conclude that the headlights and speaker(s) share a ground wire and it is weak. I removed the rear door panel and inspected everything, connections are good and the speaker works.

My questions:

Has anyone had this issue ?
If so -
Where is the ground for the headlights?
Is there anything I should replace or check?

Can someone recommend a Land Rover specialist in Montreal?
Pit Stop Garage has been closed down.

Any help would be appreciated. I'm a DIY-type and would rather try to fix it myself before sending it to my mechanic.

Thanks!