Hey that happened to me as well, I just did the heater core replacement yesterday. Only 2 of the 4 broke (the one behind the engine on the drivers side and the one to the far left on the passengers side), and the way they broke is the bolts still go through the fire wall just enough to hold it in. So, I lined it back up and just put on the 2 bolts that didnt break. It seems to hold fine, though the air conditioner is there to sort of holding it in as well. Should be fine, best of luck.