Discreet Surround Sound?

So I just got my new 08 Yukon XL last week, and for the most part, we absolutely love it. I have the SLT-2 version (I think; it's the trim level just below the Denali) with the 9-speaker Bose system and all of the navigation/entertainment stuff. Well, I was pretty excited that I was finally getting DVD-A in the car, and at the very least, DTS surround music for the car. But I found out last night as I put in a DVD-A that there was at least 1 channel completely missing. So I tried the THX setup program on one of the Pixar DVDs, and it seems that I'm not getting any surround sound at all. The rear channels still came from the front speakers, the DTS track on the DVD-A sounded very muddy because it sounded like all of the channels were coming through the front.

So was I wrong for expecting discrete surround sound?

The stock system doesn't have a processor for true surround or discrete channels. I haven't come across any oem a/v system able to provide 5.1 or 7.1 decoding besides the Lincoln MKS (which is available with THX certified 5.1 system). There are aftermarket processors specifically for the automotive enviroment you can add to get true 5.1 or 7.1 surround, but they are expensive, plus there's the labor and equipment cost of remodeling the existing speaker locations and audio components to get a proper and quality sound stage and imaging set-up.