The Bomb Shelter
Raider HQ location is found in the map of Bishop found in his room.
BUG ALERT: never leave your car at the back of the raider base, it'll be gone when you get back.
just get in there and kill everyone, then loot their stuff. You'll find 3 dogtags here, examine them. Then use the combination in the tags to open the vault. You'll find some stuff and an account book detailing deals with Bishop and NCR. Show this to the First Citizen.