I believe the BCM is located on the passenger side of the console, near the firewall in the footwell. You'll probably have to pull the glove box door and trim, passenger side lower dash trim panel, and passenger side console trim to access it.

The HMI module is attached to the passenger side lower dash trim panel, so be careful when removing it. You have to lower the footwell trim panel and then either unclip the HMI module from the trim, or disconnect the HMI module and remove it with the footwell trim.

You said the car was rear ended... before pulling the BCM I'd inspect all the wiring at the rear of the car and make sure you don't have any popped fuses in the rear fuse block and also check for any pinched/crushed/shorted/cut wiring going to the taillights, license plate lamps, reverse lamps, trunk latch, or rear parking sensors / cross traffic alert sensors-- pretty much anything in the rear harness that communicates with the BCM. Also check all the connections and cables at the battery. The BCM supports shorted circuit and open circuit fault detection on everything it controls and should still communicate even if there's an issue in the rear harness from the accident, but it's worth checking.

If it does need a BCM, they're cheap, but you will need GM SPS to reprogram and initialize the new BCM.

 

I can't help with the BCM location although I do think it is upfront near the drivers footwell or possible on the passenger side as tta1456 says. However I just had my BCM replaced and when I got the car back the right side of the rear seatback was down and there were obvious boot marks around that area. So something that communicates with the BCM is in the rear trunk compartment on the passenger side.

 

I hope you pointed that out to your SA.