GMC 2007 Sierra 1500 Pickup Owner's Manual page 522

3. Get the vehicles close enough so the jumper
cables can reach, but be sure the vehicles are
not touching each other. If they are, it could
cause a ground connection you do not
want. You would not be able to start your
vehicle, and the bad grounding could damage
the electrical systems.
To avoid the possibility of the vehicles rolling,
set the parking brake firmly on both vehicles
involved in the jump start procedure. Put
an automatic transmission in PARK (P) or a
manual transmission in NEUTRAL before
setting the parking brake. If you have a
four-wheel-drive vehicle, be sure the transfer
case is in a drive gear, not in NEUTRAL.
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Notice: If you leave your radio or other
accessories on during the jump starting
procedure, they could be damaged. The repairs
would not be covered by your warranty. Always
turn off your radio and other accessories when
jump starting your vehicle.
4. Turn off the ignition on both vehicles. Unplug
unnecessary accessories plugged into the
cigarette lighter or accessory power outlets.
Turn off the radio and all lamps that are
not needed. This will avoid sparks and help
save both batteries. And it could save
the radio!