72 duster MSD

have a 72 duster and have installed a Mallory distributor and MSD 6AL box
Currently the car doesn’t have a ballast resistor and if I wire both red ( big red and little red) together to the battery all works great except I can’t shut it off ( I temporarily installed a toggle switch) but if I hook little red wire up to 12 volt switched, when cranking power drops to low to start it. Need some help where can I pull power from to run MSD while on the key switch