GM Service Manual Online
For 1990-2009 cars only

When the ignition switch is in the RUN or the START position, voltage is supplied from the NSBU fuse to the park/neutral position switch. When the transaxle is in the REVERSE position, voltage is supplied to the left and the right backup lamps. The backup lamps are permanently grounded. When voltage is applied to CKT 24, the backup lamps are on.