Electrical - Roof light

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Can't say what it is, but try this to narrow it down.
check all the connections to the switch and relay. Hard to say if the dealer wired the switch as a ground side or hot side switch, usually, the ground from terminal 85 of the relay goes to the switch, and when the switch is closed the ground circuit is closed, activating the relay. Wiring for most relays has the power coming into terminal 86 of the relay, ground on terminal 85, direct battery power on terminal 87 and power to the lights on terminal 30. So, if you jumper terminal 87 to terminal 30, and the lights come on, it is most likely the relay. Besides, relays should not buzz, they should click when activated and unclick when unactivated.