Ford 2003 Mustang Workshop Manual page 827

Measure the voltage between ignition switch C250 Pin B4,
Circuit 1050 (LG/VT), harness side and ground; and between
ignition switch C250 Pin B5, Circuit 1050 (LG/VT), harness side
and ground.
Are the voltages greater than 10 volts?
E2 CHECK THE CONTINUITY OF THE IGNITION SWITCH
Carry out the ignition switch continuity test. REFER to Wiring
Diagrams, Cell 149, Component Testing.
Does the ignition switch test OK?
E3 CHECK CIRCUIT 16 (RD/LB), AND CIRCUIT 33 (WH/PK)
Connect: Ignition Switch C250.
Turn ignition switch to START.
Does the starter engage?
Component Test
Ignition Switch Continuity Check
Refer to Wiring Diagrams Cell 149, Component Testing.
Multifunction Switch
Refer to Wiring Diagrams Cell 149, Component Testing.
No
REPAIR the circuit in
question.TEST system
for normal operation.
Yes
GO to E3 .
No
INSTALL a new ignition
switch. REFER to
Ignition Switch . TEST
the system for normal
operation.
Yes
REPAIR Circuit 16
(RD/LB). TEST the
system for normal
operation.
No
REPAIR Circuit 33
(WH/PK). TEST the
system for normal
operation.