Iraqvet05 wrote:
From the 2009 Silverado manual page 4-79.
"The fuse for
the battery feed is not required if the vehicle has
an auxiliary battery. If the vehicle does not have an
auxiliary battery, have your dealer/retailer or authorized
service center install the required fuse.
If charging a remote (non-vehicle) battery, press the
tow/haul mode button located at the end of the
shift lever. This will boost the vehicle system voltage
and properly charge the battery. If the trailer is too light
for tow/haul mode, turn on the headlamps as a
second way"
I'm assuming since the OP has a Duramax there is a second battery installed and the fuse is not needed like it is with the gas engine Silverados. It also appears that the charge wire is only energized (somehow) when the tow/haul button is on or when the headlights are fully on (not just the DRL's)...it is not a constant hot 12V.
It provides voltage without the tow/haul engaged, but engaging tow/haul or turning on headlights will signal computer to provide higher voltage from alternator, improving chances of high enough voltage and current to reach the trailer battery and make a difference.
