When the ignition switch is in the RUN position or in the START position, voltage is available at the backup lamp switch from TURN B/U fuse 16 through CKT 139 (PNK). The backup lamp switch is part of the park/neutral position switch. When the transmission is in the reverse position, the backup lamp switch contacts close. Then voltage is available through CKT 24 (LT GRN) to both backup lamps. Voltage is also available to trailer wiring provision connector C406, as equipped. The left backup lamp has ground through CKT 150 (BLK) to ground G400. The right backup lamp has ground through CKT 250 (BLK) to ground G401. The transmission in the reverse position applies voltage in order to illuminate the lamps. Shift to another gear or stopping the engine extinguishes the lamps.