Ford 2003 Mustang Workshop Manual page 3455

If the high mounted stoplamp is inoperative, measure the voltage
between the inoperative high mounted stoplamp C475 (C4199 for
Cobra) pin 1, circuit 569 (DG), harness side and ground while
pressing the brake pedal.
Is the voltage greater than 10 volts?
H2 CHECK CIRCUIT 9 (LG/OG) OR 5 (OG/LB) FOR AN OPEN
Disconnect: Multifunction Switch C202b.
Measure the resistance between the inoperative stoplamp
connector and multifunction switch C202b pin 9, circuit 9
(LG/OG) (LH), or C202b pin 5, circuit 5 (OG/LB) (RH).
Is the resistance less than 5 ohms?
PINPOINT TEST I: THE STOPLAMPS ARE ON CONTINUOUSLY
Test Step
I1 CHECK THE BRAKE PEDAL
POSITION (BPP) SWITCH
Key in OFF position.
Disconnect: BPP Switch
C278.
Do the stoplamps continue
to illuminate?
Result / Action to Take
Yes
GO to I2 .
No
INSTALL a new BPP switch. REFER to
Brake Pedal Position (BPP)
operation.
If rear stoplamp, GO to
H2 .
Yes
INSTALL a new
multifunction switch.
REFER to Section
211-05 . TEST the
system for normal
operation.
No
REPAIR the circuit in
question. TEST the
system for normal
operation.
Lamp Switch—
. TEST the system for normal