Highest concentration over the area Tim was sitting, and the second highest concentration over the driver's side. Unless MS was a magician/contortionist, it's pretty clear now he did not fire a gun from the back seat which had the lowest concentration of GSR on the roof liner.
MS would have had to hold the gun in his left hand in order to shot from the back seat of TB's truck. We do know
DM is left handed 
. The struggle in the truck...I think TB saw DM pull the gun out of his man purse. Andreww has speculated DM was driving when TB was shot. Why do you speculate that Andreww, just curious?
OK so this is what I speculate happened. DM left his man purse with his gun in it, in the Yukon, parked in the field. The three left TB's laneway in TB's truck and as they drove along, one of them spotted
"this strange Yukon just parked in a field", thought it was suspicious and suggested they check it out. After all, we know how people steal vehicles, take them for joy rides and then just abandon them kwim

DM pulled up along side the Yukon, but facing the opposite direction of it, driver's door to driver's door. MS got out of TB's truck, opened the Yukon's driver's door, fetched DM's purse and gave it to him through the driver's side window of TB's truck. DM pulled the gun out of his purse and TB saw it. There was a brief struggle and DM shot TB. TB probably wasn't even ordered out of his truck and didn't have much of a chance to react, other than his first thought would have been to protect himself from being shot. It was a fight or flight situation and TB instantaneously chose to fight. DM didn't care if he blew out the passenger side window and made a mess as he knew the truck could be cleaned and the window and seat replaced. You can get all kinds of them at the auto wrecker for a decent price

. To DM it was worth the thrill and the bit of money he would have to spend to replace a window and seat. After all, he got a $25k truck for nothing. I bet he didn't realize the seats were joined and he'd have to replace both.

JMO.
DM is the one who allegedly shot his own father. That to me speaks volumes. When someone has the audacity to murder their own father because of greed, he was willing and eager to do it for a truck also. MS was his lost puppy who just followed him and helped him for a chunk of change. IMO DM was the shooter. Hopefully we will hear if there was any blood evidence found in the Yukon. If not, or if very minuscule amounts were found, to me that would suggest MS was not in the back seat of TB's truck when TB was shot. And we know these two aren't the brightest crayons in the box, an order of fries short of a Happy Meal, but I really don't think DM would have trusted MS with his gun, to fire it under DM's nose (if MS was outside the truck, on the driver's side) while aiming the gun at TB. But that's just ALL MOO.
Bottom line is, I hope they are both found guilty of first degree. Premeditation. MOO.