I had the same problem my Dart would`t shut off at the switch, I ran a new wire directly from the alternator to two relays to the switch and problem solved, it also starts easier and the battery does`t need charging as often as it used too.
68 Dart 410 / 904 92 D150 original owner 21 Ram 1500 Quad Cab, Big Horn , Hemi ,4x4 23 Audi Q5 16 Honda HRV