I was curious of anyone knows how to read this. On the "circuit board" under the bumper it makes sense that Power is supplied on C1 with ground.
C2 is the left lights and C3 is the right.
On the LT GRN feeding the + voltage, what does the "C" mean. I would have assumed wire C but as shown in the connector pin outs it's on B. And the LT GRN colored wire is to the B pin.
It really doesn't matter, as I'm still tracing the LT GRN, but I was just curious if anyone knew.

 

C usually designates that it is a Connector interface. or they have the connector blocks named in an alphabetical order. if the diagram has a legend it would explain the abbreviations it uses.

 

Had a quick look.

The Rear Junction Block (RJB) seems to have connectors soldered to a PCB inside an injection moulded plastic-block (see below, ignore the white connector)



Here's an annotated view of the block, so far I've not found references to C4 yet (if that's what its referred to, may be trailer connector?)


The diagram is indeed a little weird, but essentially as you say, C2 is Left Lamps, C3 is Right Lamps

C1 are the Control Signals:-
BCU-Power-Control IN (C1-C) and Ground G401 on (C1-E) [plus other lamp signals]

C2 are the Left Lamps:-
Left Back-Up Lamp, "C2-F" = BCU +12V (when in reverse), and "C2-B" = OV (always) [also "Left Tail/Stop, Left Turn]

C3 are the Right Lamps:-
Right Back-Up Lamp, "C3-E = BCU +12V (when in reverse), and C3-H" = OV (always) [also Right Tail/Sop, Right Turn]

The Black chassis Ground wire (C1-E) is probably OK as the Tail/Stop Lights and Turn Signals also share this ground, that's not to say the block itself is not faulty internally.

To check the block's internal ground connections, unplug all connectors and measure the resistance between "C1-E" (common) and all the following pins:-

C2-A (Left Tail/Stop)
C2-B (Left Backup-Lamp)
C2-C (Left Turn)
C3-C (Right Tail/Stop)
C3-D (Righ Turn)
C3-H (Right Backup-Lamp)

They should all measure very low resistance, less than 0.3Ω (approx) plus they should all connect to each other (before measuring short your meter leads 1st to check the leads residual value)

Re-connect C2 and C3 (only) and measure the resistance between C1-C and "Chassis" it should around 10Ω ish (low ohms), these are the Back-Up Lamps filaments wired in "parallel" by the block.

If the above grounds measure OK and C1-C has low ohms, re-connect C1 and measure the voltage between C1-C (if you can) and Chassis, it should be 12V'ish whilst in reverse and in this case the lamps must be ON

You mentioned "C" which is "C1-C" but I think the "B" you refer to is in a different connector i.e. "C104-B" but still relevant.

C104 seems to have two connectors and appears to be inside the engine compartment, below the brake reservoir, with the harness going to the rear of the vehicle (as far as I can see)

The Back-Up Lamp circuit also connects to Trailer connector pin A, it may be worth measuring here, you could always connect a lamp between pins A and B(GND) and see if that works, at least you'd know the BCU is working.

It seems the L-GN wire (or LT GRN) also goes to I/P Junction Block C1 (Front Right Passenger Side, behind glove compartment may be ?) but I have no further details on this.

There is also a 68-Way connector to the Relay Block (I/P C1) connector P/No 12193905, this is a massive connector, I think it's LHS of the driver's area.

It's seems the back-up lamp circuit goes everywhere

Note, be careful not apply any external voltages to the block with C1 connected, not sure what this could do to the BCU etc. pin A1 (L-GN) circuit 24

That's all I have at the moment, note I've never looked at any of this before so my info is a bit speculative ...

 

Today is my last day on this... I don't know why GM bothers to create a service manual.
Per the wiring diagram and going to the lower right hand dash panel there is supposed to be a 12 pin connector as shown.
There is a 12 pin but the locking tab is not where it's shown. Also there are no green wires in it.
Next to it is a 6 pin with 3 wires and 2 are green. No voltage there while in reverse, but I don't even know if that's the right wires.
Anyway, the next place is the BCM, and I don't even want to touch that over just reverse lights. Not to mention there is some extra stuff in that side from what appears to be an alarm and maybe at one time remote start.
I just think the deeper I dig the more my chances are of really messing up something important.
I think my next step will be to just wire the trailer backup circuit to the truck backup lamps.
Since that's a 20A fuse I might change it to 10A to match what the car's circuit has.

 

one question have they worked on your transmission in the resent past as the reverse signal would be commanded by the range selector signal. does the cluster say that your in reverse?

 

Thanks for the reply... yes it goes to reverse and the trailer reverse shows 12v in reverse.

 

My suggestion:

1. This all started with LED bulbs not working. So jam in a regular incandescent bulb, and go on your way. That was easy
2. Clean up the electrical mess leftovers from remote start, aftermarket car alarm, and anything else that is not supposed to be there. Those things always cause problems down the road.

There! I fixed it for you!

 

Regular bulbs don't work either. They may not have been working for a while.

 

Got it. OK, I stand corrected.

 

Don't think C104 plugs into the I/P Junction Block (see below and attached pdf) ?

The I/P Junction Block 12-pin connector is C4 (Brown) and you'll notice there no Back-Up wire anywhere in the PDF which contradicts the wiring diagram.
This could be a schematic error, if you interpret "I/P Relay Block" instead of "I/P Junction Block" all the circuit references are correct i.e. C3-A and A1

If you search for "Backup" in the attached pdf you'll find more references, e.g I/P Relay Block C3-A L-GN

The relay block is in the Driver's side footwell and very easy to get to, you may have more luck there