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These lights are all computer controlled ! Several modules involved DIM - dash integrated module , RIM - rear integrated module .They share information on serial data communication network . Class 2 an or GMLAN !
The rear fuse block supplies battery positive voltage through the REVERSE LAMP fuse to the coil and switch terminals of the reverse lamp relay. The DIM sends a power mode message to the RIM indicating that the ignition is in the ON position. When the driver of an automatic transmission equipped vehicle places the gear selector in the REVERSE position, the powertrain control module (PCM) sends a class 2 message to the RIM. The message indicates that the gear selector lever is in the REVERSE position. When the driver places a manual transmission equipped vehicle in reverse the reverse gear position switch mounted on the transmission closes. This provides a ground for the reverse gear position switch signal line, informing the RIM that the transmission is in reverse. For both automatic and manual transmission vehicles the RIM then grounds the control circuit of the reverse lamp relay . This energizes the reverse lamp relay, closing the switch side contacts and providing battery positive voltage to the backup lamp bulbs. The backup lamps are permanently grounded at G402. With both power and ground the backup lamp bulbs illuminate. Once the driver moves the shift lever out of the reverse position a message is send either by class 2 by the PCM or the reverse gear position switch opens causing the RIM to remove the ground from the reverse lamp relay control circuit. This opens the contacts in the relay turning OFF the backup lamps.
This is something best left to the dealer or a ASE certified repair shop . Need a factory or professional grade scan tool to diagnose an repair this . http://www.public.asu.edu/~pheanis/documents/VBI_CAN.pdf
My guess would be the neutral safety switch on the transmission. The switch also acts as a backup light switch when the shifter is in reverse. I would check for power at the switch for the reverse light circuit.
Check if you are getting power to the reverse light switch. Turn ignition on, disconect terminals to the switch and check for power on the positive terminal using a light bulb or a test meter whatever. If power is there jumpthe 2 terminals and check if light backup light works. If it does replace switch. If ther is no power at switch check wiring from fuse box. If light does not work when switch is jumped check wiring to back.
it depends if it is auto or manual. There is a nuetral saftey switch on the auto-located on the manual valve arm(the piece that the linkage from the shifter connects to at the trans.) On the manual trans. there will be a 2 pole switch on the top shift plate that is for reverse only. The starting(clutch engagement switch)is located at the clutch pedal-2 seperate switches. First check for power at the back up light bulb connecter with reverse engaged(may have power at all times when key is on). Then check for ground when reverse is engaged-from bulb connecter to ground) with reverse engaged. The switch will just provide the ground when reverse is engaged. The power comes from an alternate source. Most likley a switch issue but at least this way you can be sure.
If there is no power at socket, check for a blown fuse, if fuse is good still check it for power, you can check the b/u switch on the trani, I think your is part of the neutral safety switch,do a wiggle check( might be able to see a loose connection) do it with key on wheels chocked and/or E-brake applied,and the trani in reverse. the truck does not have to be running. Just the key in the run position. and you can check for continuity(good ground) at socket. Good luck.
Not sure what transmssion you have but basically it goes from fuse on P/O wire to either the MLP switch on automatic or just a backup lamp switch on manual and leaves the switch on BK/PK wire to the bulbs. Should be easy to trace with a 12v test light.