Manaz
1000 Posts Club!
that's what I figured from the pictures. I'm no expert, but a trunk fire to me screams electrical. I've known a few cars personally to burn to the ground due to crappy aftermarket stereo installs, namely subs/amps in the trunk. If Kia is giving him a hard time on this, I bet there was some aftermarket work back there they're trying to place the blame on.
Remembering the battery is in the boot/trunk in the Stinger. Maybe not the cause in this case, but battery fires aren't exactly unheard of in the automotive industry.