Duh, uh, the solenoid. Where would that be located, and which post on the light switch? Sorry, I am pretty green about these matters.The alt wires coming out of the new engine usually are different than the one you are replacing, so some improvising is usually needed. If your new setup is charging ok, just run a wire from the fuse,or run a wire and fuse from the hot post on the solenoid to your light switch and that will do it.
Thanks.
Caruso