This upgrade may not be chosen for open-topped vehicles. The vehicle does not have any crew: instead the vehicle is possessed by a Daemon, hatches are all welded shut and sealed with sigils. Runes of the Chaos power that has possessed it will be prominently displayed all over the vehicle's hull. If the vehicle had a troop carrying capability then it can no longer carry any troops. The Daemon controls the vehicle and may ignore the effects of any 'stunned' and 'shaken' results on the Vehicle Damage tables – such damage has no effect on a Daemonically possessed vehicle.
Daemonic Possession
Source: Codex: Chaos Space Marines (1999), p. 21