GMC 2015 Sierra Denali Owner's Manual page 261

9-82 Driving and Operating
If the vehicle is equipped with the
heavy-duty trailering option, see
Heavy-Duty Trailer Wiring Harness
Package earlier in this section.
When the camper-wiring harness is
ordered without the heavy-duty
trailering package, a seven-wire
harness with a seven-pin connector
is at the rear of the vehicle and is
tied to the vehicle's frame.
Electric Brake Control Wiring
Provisions
These wiring provisions are
included with the vehicle as part of
the trailer wiring package. These
provisions are for an electric brake
controller.
The harness should be installed by
your dealer or a qualified service
center.
Tow/Haul Mode
Pressing this button at the end of
the shift lever turns on and off the
Tow/Haul Mode.
This indicator light on the instrument
cluster comes on when the Tow/
Haul Mode is on.
Tow/Haul is a feature that assists
when pulling a heavy trailer or a
large or heavy load. See Tow/Haul
Mode on page 9-39.
Tow/Haul is designed to be most
effective when the vehicle and
trailer combined weight is at least
75% of the vehicle's Gross
Combined Weight Rating (GCWR).
See Weight of the Trailer under
Trailer Towing on page 9-71. Tow/
Haul is most useful under the
following driving conditions:
When pulling a heavy trailer or a
large or heavy load through
rolling terrain.
When pulling a heavy trailer or a
large or heavy load in
stop-and-go traffic.
When pulling a heavy trailer or a
large or heavy load in busy
parking lots where improved low
speed control of the vehicle is
desired.