Sorry, thought I posted in the Firebird forum. I'll leave reply there in case they are still similar as GM used them across the board. They also commonly had a free spinning washer on the nuts that was caulked up. So if this doesn't apply, ignore.
#7583551 sealing washers were inserted between the spoiler and the trunk lid over each stud.
Here's the reproduction version of the rear spoiler fastener kit, they decided to make smaller ones for the end nuts.
https://momcp.com/wp-content/uploads...04/524-012.jpg