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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 1660
Experience:  GM Grand Master Technician 2007. 14 year experience.
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Have 06 GMC 2500 with flatbed and after mkt tail lights.

Customer Question

Have 06 GMC 2500 with flatbed and after mkt tail lights. Problem is no break lights or turn signals and no apparent pwr to fuses in cab. Just replaced L side harness to taillight and had replaced jct box last year. 25 amp fuse under hood is ok and trailer 7 pin lights all work. Must be a separate pwr source or relay to feed rear lights, but cannot identify any other fuses to check or if there is even a relay. Can you outline a plan to sort this out?
Submitted: 5 months ago.
Category: GMC
Expert:  Kevin Bourcier replied 5 months ago.

hi . Check on the rear cross member to see if your truck has the IC lighting module

Customer: replied 5 months ago.
Not sure how to identify the IC module, but the junction box is there. It is a flat panel with 4 colored coded male connectors and 3 are in use: one the main feed and the other two going to the L and R taillight assemblies.
Expert:  Kevin Bourcier replied 5 months ago.

okay check these fuses under the dash

Customer: replied 5 months ago.
Dwg is not clear but sure its the fuses marked; Lt/ Rt trn and Lt/Rt trn. No pwr w/ probe with key on or off, but with flashers on, all fuses are hot. So to recap, all left tail light wires disconnected and no turn, break or haz voltage to wires. Right side has all but break light working including turn signal. Trailer harness has all working. I was confused at first writing as I thought I should have power at fuses without turning haz or turn signal on. Could I have a problem at the brake switch?
Expert:  Kevin Bourcier replied 5 months ago.

the module at the back of the vehicle is where I would check for power on the plug coming from the front of the vehicle . To be honest the standard practice when a vehicle does not have a conventional " pickup box " on it and a flat deck as in your case is to wire the flat deck lighting to the trailer harness

Customer: replied 4 months ago.
Just to be clear. If I pull the feed connector to the junction box at the rear and check the terminals using my power probe with ground to the battery, I should be getting 12 volts for each of the taillight functions. If power is present, then problem is in box or harness downstream. If not, then problem is in brake switch, fuse or relay? Right?
Expert:  Kevin Bourcier replied 4 months ago.

if there is an issue at that connector ( no voltage ) you may still have possibly have an issue in the multi function switch ( turn signal switch ) but check at the rear connector first .