When you rotate the lock cyl it sends a low current
voltage through the neutral safety / clutch interlock switch
to the starter solenoid/relay . The low current energizes
a magnetic winding in the starter solenoid/relay to close the
high current circuit from the + battery cable and completes
the path through the commutator to the motor windings
which is grounded through it's case to the engine via it's
mounting bolts .
The low current wire on a ford is usually Red/Bl or Red/Gr
and connects to the S terminal on the solenoid or terminal
86 on the relay , depending on which system you have .
The pos battery cable can be red , blue or black .
The real trick of course is tracing the circuit down
the steering column and through the various joints and splices
in the wiring harnesses on it's way to the solenoid/relay .
The high current circuit being the Pos battery cable is much easier .
Or you could spend the $10 for the manual with the wiring diagrams
And pictures of the circuits in question