Did it come with the switch ? The beacon switch has it's own ordering code (RPO) Do you know if the plow group includes the RPO ? If not you'll most likely need harness and all.
I have that switch factory on my d-max when I put my cab lights on I had the head liner down and seen two wires in between the cab and head liner. It clicked in my head that they could be for the beackon light switch so I put a test light on em and one is a ground and one is a hot when the switch is turned on. So to sum it the the wires for the switch are under the headliner
I would say it does have the switch because why would he be lookin for the wireing harness for a switch that isn't there. Plus he said it had the plow package and the beacon light switch was included in the snow plow prep group.
Yeah mine also does the clicking sound when u push it when it was quite in my truck u used to push it to mess with people lol. But yeah your wireing harness is goin to be under the head liner pull it down in the front and look back in there u will see em. Are you hookin up a beackon light or using the switch for somthing else