[MikeD]

In the individual vehicle screen (accessed by hitting the space bar when the vehicle is clicked on) you can activate or deactivate individual weapons by clicking on them.
Active weapons are yellow inactive are blue.

But it's not a very subtle or time effective way to do things, since you have to go back before ending the turn and reactivate weapons you might want to use to defend yourself if a bunch of enemy infantry show up on their turn.

Is there a more simple elegant way to do this?
Mike