fast ez efi

I have a 68 cuda convertible and a 72 dodge dart. I installed efi into both cars they work well other than I am having trouble finding a switched power wire that delivers 12 volts when the car is cranking over. Can anyone suggest which wire I should use for my switched power. This wire I will use to trigger my relay to deliver my full twelve volts to the fast computer
Thanking you in advance
Brian