The window could have been rolled down if the door was closed - or - perhaps the shooter hopped out of the truck and fired at TB with the driver's side door open.
All MOO.
IMO this is a very likely scenario. If they drove into the lane on the pretense of relieving themselves, left Tim in the truck, then came back, keeping the gun( which was carried in a small canvas satchel ) hidden perhaps behind the shooter's back, opening the driver's side door, and taking Tim by surprise, in the dark. Suddenly DM or MS reaches in, and shoots him, possibly in the neck, then again once or twice in the head, ( thrill kill, and wanting to make sure he is dead) accounting for the massive blood loss, and high GSR count in the truck ( the highest the expert had ever seen ) Passenger door and window were closed, so a slight breeze might come in from the open driver's side door blowing most of the GSR to the passenger side interior roof, but also a substantial amount getting on the driver's side interior roof. At least one bullet passes through TB, and shatters the passenger side window. One to three shell casings are ejected into the back seat. They miss one in the clean up.