It seems there is a peck of confusion out there concerning exactly how the “base platform” weapons on Imperial heavy cavalry units actually function, and even are mounted (including at least one case of confusion so profound as to believe the rear/local defense guns were “sticking out the back of the turret”, in the style of anti-infantry defense MGs from early last century, despite the platform – without a module installed – not having a turret.).
Here is a diagram in my inimitably terrible style:
That’s your base platform, driving left to right. Green at the front are your cheek-mounted (i.e., in a three-axis gimbaled mount on the side of the vehicle) heavy mass drivers, target designators, and micromissile launchers. Purple at the rear are your cheek-mounted medium mass drivers for local defense. Both weapons are illustrated in their default rest position, i.e., forward-facing or rear-facing, respectively.
As can be seen from the shaded fields of fire, both can train sufficiently to hit anything on their side of the vehicle that doesn’t actually involve training through the platform body or the other weapon mount; i.e., the forward cheek-mounts can hit anything from directly forward (with a small blind spot directly in front of the vehicle) to not-quite-rear; and the rear cheek-mounts can hit anything from directly behind (with small blind spot directly behind the vehicle, likewise) to not-quite-directly forward.
In short, there are plenty of things for them all to shoot at.