Another Brake Light Issue

[Snoobly]

Hey All,
We are owner of a new to us 2006 Volvo XC90 since Sept. of 2019. I have been tackling issues as they come up by using the forums such as this one, so I thank everyone in advance.
Now to the good stuff.
2006 Volvo XC90
2.5T AWD
145,000 miles on the odometer.
Stock except for Kenwood touch screen radio that works excellent.

I am no novice to maintaining and fixing my own vehicles, but I am new to the Volvo world. I have a garage full of tools and diagnostic experience under my belt from previous and current projects.
The issue I am having is the famous brake light issue where the two lower brake lights do not light up and sends a message to the display unit in the dash that reads "check stop lamps" or something along those lines. The auxiliary (top) lamp does light up.
First instinct was to check the light bulbs, all are good. They are even the silver metal instead of the bronze which I have read is needed since Volvo can be picky.
Next was the fuses. I printed out the entire wiring diagram for the 2007 (I couldn't find a 2006). I checked every fuse that could have something to do with brake lamps.
Next came research where I found that the 2004 and older had a relay that frequently went bad. In 2005 Volvo switched to the transistor style REM where everything is internal. I hooked up my scanner to the car and checked all the computers for any errors. CEM and REM both read fine. I can go into the scanner and activate brake lights from there (no pressing the pedal) and the top light comes on, but not the bottom ones. No message on the dash display when activating the brake lights this way though.
I went to the junk yard and swapped the REM, just in case. Still same diagnosis.
I am pretty stumped.
Does anyone have any pointers or previous experience on this that isn't posted on the forums?
The zip file is a few pictures of the wiring diagrams that I have been referencing trying to figure all this out.

Volvo.zip

Photo of the wiring diagrams for diagnosing

(9.32 MiB) Downloaded 143 times

[RickHaleParker]

The CEM powers the lower brake lights. The REM powers the third brake light.
One of the bulb sockets maybe shorted to ground. Check for that first.

Start at CEM A:15 check for the brake light signal voltage, trace the route below until you find no signal voltage. ( With the brake pedal pressed and key on)

Check for the lower brake light voltage at the highlighted points.

1. CEM A:15 to REM A:18
2. REM A:18 pass through to REM B:3.
3. REM B:3 to lower brake lights (Both sides).

[Snoobly]

Thanks Rick, I'll try that. I was trying that with the scanner tool and I wasn't getting any signal on those. I will try it again with the actual brake switch (manually pressing the brake pedal). Should the voltage be 12 Volts?
Thanks again!