switched acc in 2005 chevy express 3500

I've got a 2005 Chevy Express G3500 15 passenger van. I'm looking to install a switched 12V accessory outlet towards the rear of the van on the driver's side. Where would be the best place to get the power from? I found a document on the GM Upfitter site that mentions that there is a switched power junction behind the driver's B pillar, but I can't find it. I'm thinking that it might only be on the chassis cab models, not the passenger models. If switched power is too difficult, constant power would be ok, although I would prefer switched. Thanks in advance