I just went and looked at my truck and there is if fact a pushbutton at the end of the light stem. Didn’t even notice this. The owners manual does not even mention or show a button at the end of the light stem.
From owner's manual page 141 (on PDF file)
High-Beam Systems
IntelliBeam System
If equipped, this system turns the high-beam
headlamps on and off according to surrounding
traffic conditions.
The system turns the high-beam headlamps on
when it is dark enough and there is no other
traffic present.
Turning the IntelliBeam System On and Off
To enable the IntelliBeam system, turn the
exterior lamp control to AUTO or 2, then
press b on the turn signal lever. If it is not
dark enough outside, the high beams may not
turn on.
Depending on the vehicle model, the
following displays when the IntelliBeam
system is enabled:
• b displays on the instrument cluster
• Auto High Beam Active message displays in
the Driver Information Center (DIC)
To disable the IntelliBeam system, turn on
the manual high beams or use the Flash-to-
Pass feature. See Headlamp High/Low-Beam
Changer 3 143 and Flash-to-Pass 3 143.
More in the manual.