Where are the battery grounds on 08 Duramax?

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My grandson enlisted in the US Army and is in training at Ft. Benning GA.  I'm storing his 08 Silverado with the Duramax engine until he can come home and get it.  I'd like to install a battery disconnect so that he can disconnect the batteries when he's on maneuvers or otherwise away for short periods.  I have a disconnect switch capable of handling 500 amps continuous load that should well for this truck.  I can't see where the ground cables on the batteries are grounded to the frame - there isn't much room to see especially on the passenger side battery.  Can someone give me some idea where the cables are grounded on an 08?  I'm waiting on the wire and hardware to do the job and a little advance knowledge would be a great help.  Thanks in advance!