How was he murdered?

But if he shot him while the top of the car was down, they should've found it that way when found dead.

Nero, you are wrong, the top of the car was obviously down it didn't need to be a sniper. The gunman simply shot the person when the top of the car was down.

The man was shot from a high and long distance. Most likely by a sniper who, why not, was on the roof of a high building, since you can't shoot someone from a convertible without leaving a mark on the window.