Battery drain

If all else fails check your wipers and make sure they are parked. I had this problem for years and went through several alternators and even replaced a new battery. The 68 at least has to have full rotation of the wiper arms. If someone had hit the switch with the wiper door closed It would allow the wipers to activate power and not be able to park themselves. 

Open the wiper door (override switch) and switch on the windshield wiper switch and then turn them off and make sure they park to get the power drain off.