GM Service Manual Online
For 1990-2009 cars only

Voltage is applied continuously from the B/U LP fuse to the PARK/NEUTRAL position switch through CKT 840. Voltage is applied through the closed contacts of the PARK/NEUTRAL position switch to the right and left backup lamps through CKT 24, when the transaxle is shifted into the REVERSE position. The backup lamps are grounded through CKT  350 to G302.