I spent a lot of time on that one. Seems like having few trying to make the same mission is bugging it or so. At least I couldnt finish it yet, once first one doesnt work, then last one doesnt work. Restarted 20 times.
Biggest problem is the mobs seem to spawn infinitely. I would clear out the raiders, suddenly 50 (im not even lying) hellbugs spawn, the whole map is flooded with red dots, two of the grenade launcher big mobs spawn, and then the huge raider that sets himself in rage with grenades spawns (or two of them). Seriously the whole map becomes red and I'm pretty much alone trying to fight them off.
Eventually I clear the whole place, maybe am lucky and the turret works, move on to the next and exact the same thing happens. I finished the 12 grenades from the biggest raiders contract cause they just keep spawning all the time, all while just trying to finish this one quest.