'66 Stang 289 starter issues

Your negative cable on the battery is either bad or not making a good connection, it should be connected to a bolt on the block on passenger side in front of the motor mount (don't just do a visual inspection, wires can go bad inside the crimp), this is your problem, or you have a bad starter drawing too much current (less likely). And anytime you start your car, remember, the starter must cool, so if you crank the engine say for 30 seconds and it doesn't start let the starter cool for 5 min before you try again. And you should also have a ground wire on the passenger cylinder head to the firewall, this is a body ground (it may be different on a 66). Good Luck.

Rex,

A '66 is as you described it. I also agree with your assessment; the battery isn't grounded to the block good enough so that the return current is going through the small wires near the voltage regulator to find a ground. Those wires are tiny in comparison and will burn easily with that kind of current. If the block is grounded correctly starter current will not being going through those small wires.