Like a bad ballast resistor... but not.

My 1972 Plymouth Duster, slant six 225, is acting like it has a bad ballast resistor. I turn the key, it turns over and wants to run, but as soon as I turn it back it dies. So I replace the ballast resistor, because that's what it sounds like, but it keeps doing the same thing. Shorting across the resistor doesn't do it either.

Where's the switch/relay that jumps it to 12v for starting and back down through the ballast resistor for 6v running? Do those go bad?
Thanks
xander