IIRC there was some discussion about this at some point, in which Demian indicated he had decided against the use of open-topped vehicles.

Some of the rules level complications I see in allowing an exposed passenger space:

[*]ranged combat: penalties for firing from vehicle or not? can all passengers fire at the same time?[*]close combat: do the passengers still count as part of the vehicle for this purpose? if not, how many passenger can fight back, and which of the targets (vehicles, passengers) must the enemy attack first (or can they make a pick)?[*]damage to passengers: do exposed passengers count as a vehicle target or an infantry target? do any additional cover rules apply? is the hit randomized between passengers or does either player get to select the model hit? how many models do AOE weapons hit at a time? does an AOE weapon hit both passengers and the vehicle always, or never, or randomly? how does morale in general and different personal morale and unit morale effects in particular work for the passengers?

There is undoubtedly more. Some compromises could probably be made that make some of the above problems go away, but it's still not a simple issue.

In short, I don't have any suggestions at this point. It might help the discussion if you'd draw an outline as to what level of detail you would like to accomplish.

Well, if everyone on board, including the driver, was exposed, you could treat it as a size 3 Flying infantry model.

You could also treat passengers as weapons systems and use the "exposed weaponry" design flaw.

You could also make a weapon called "hail of small-arms fire" and treat it as a cupola weapon (not so much of a stretch to imagine a deck-clearing shot from the enemy taking down all your gunners, leaving only the guys who were cowering in more sheltered bits- ie the passengers.)