Thanks for the info. "Backup" does not appear in the PDF you attached.
Best,
-Dave

 

Apologies, only the "IP Junction Block Connections.PDF" attached, here's the"Connector End Views All.pdf" "Backup" is found in six places.

 

As it happens in about the "Cabin Temperature Sensor (Aspirator)" at 4:56 shows the I/P Relay Block connector C3 which is the brown 12way connector on the right below connector C4.

The top RH pin IS "A" L-GN and the bottom RH pin IS "C" Black which is OV so easy to measure, which I've just measured and confirmed it 100% provides 12V when in reverse and goes OFF when not in reverse.

If there's no +12V there it could be your BCU or connector/wire to the BCU, if it is there, then you can rest easy but need to look elsewhere like C104 or at the rear somewhere.

So far I've been unable to locate the Trailer Backup wiring, if the Trailer output is working, it suggests the BCU is fine, so C3-A should be OK too.

It looks like the Trailer Lights somehow links-up at the rear of the truck, it may be worth looking to see where the connector goes to.

 

Maybe someday I'll get a chance to revisit this... Certainly a frustrating adventure of nonsense.
I'm owner #3 on this 2006 so I can't take credit for all the mess.
When I bought the truck I had to clean up the radio install. It's a factory head unit but several boxes behind it are not.
I found at least 3 boxes for the alarm, which works well, and I can't complain.
Then it has the wiring for the 4 light "luggage bar" and I installed a brake controller for my trailer.
But at this point, I'm afraid to chase it farther. Maybe another project will force me to dig deeper. But maybe I'll nurse this one along.
2006 with 81K miles still has some life in her, I hope.
Tonight I jumped the reverse lights from the trailer harness and it works fine. Tomorrow I'll clean up, tape and zip tie everything to look as factory as possible.
Thanks to everyone for their advice. The LED backup bulbs are super bright!

 

Indeed sometimes you just have to get things done.

My curiosity got the better of me.

The circuit diagram for a 2004 appears to be in error, "I/P Junction Block" should read "I/P Relay block" then all the references are correct.

The Trailer circuit and L-R Backup Lights are all connected to Circuit 24 so the odds are the BCU is OK, the voltage at connector C3 (as measured in my video) will be fine. So the problem is could be in the harness (C104) or most likely is the either internally in the "Rear Junction Block" or Connector C1

Anyway to test the Backup line watch

 

I would laugh if it wasn't so sad... I'm throwing away the Service Manual! Thanks for the video.