To move someone in a car/tank/plane as a driver(pilot), gunner or cargo, put the following lines in the init field of the unit:
this moveInDriver vehiclename
this moveInGunner vehiclename
this moveInCargo vehiclename
To assign a unit to a vehicle, put this in his init field:
this assignAsDriver vehiclename
this assignAsGunner vehiclename
this assignAsCargo vehiclename