Tried to replace the heater core yesterday. Nightmare. To access the heater core you have to remove the heater box on the fire wall, which sounds simple but one of the bolts you can't access because of the fender skirt. The Haynes manual calls for loosing the fender skirt to access this bolt. The problem is that the bolts connecting the fender skirt to the fender bolt into nuts that are spot welded, well half of the welds broke leaving the bolts spinning in place and you can't reach the other side to secure the spinning nuts. I know I could hack the bolts off but I would have to have new welds made on the fender. I'm pretty sure you can't remove the core from under the dash. Anyone successfully replace their core in a 69-72? I attached a photo of the heater box and one of the truck since I've never put any photos up before.