This is also the part I am struggling with. We know that TB willingly got into the vehicle, but we don't know if at any time, he was forcibly confined to the vehicle. The only people who were there were TB, MS and DM. The only way we will find out is if either MM or CN were told this information by one of the accused, and they are willing to share that information under oath. From everything I've seen/heard so far, and making an assumption or two on my own(like why would you shoot someone in a truck you were hoping to drive on a regular basis?) my best guess, and it's just that, a guess of what happened, I think perhaps they got a certain distance down the road to where no one where hear anything, DM demands that TB get out of the truck. Perhaps his plan was to drive off and leave him there, or perhaps all along it was to shoot TB once he left the vehicle. If this is what took place, then in my view, forcible confinement never took place, and unless MS knew DM planned on shooting TB, it would be hard to convict him of first degree murder.
Eta: perhaps after being asked to get out, TB refused to get out, not wanting them to drive off with his truck, thus why He was shot in the truck, not outside.
I think everything will come down to the girlfriends testimony, and more specifically to MMs testimony. To me, she is the more credible witness between the two, and. I have a feeling MS may have shared more with her than DM did with CN.
All above are MOO, and guesses as to how it happened