What you are missing in that diagram is power to the rest of the car. That can come from the starter battery wire to the start relay as in the original wiring, or some other wiring from the down stream from cutout switch. The way that diagram is, you don't really need the relay, it could be bypassed.

As far as original wire (depending on the year) it went through the amp meter in the dash, then back to the battery via the post on the start relay. Just remove it all together. BTW, you cannot use the amp meter with this wiring.