I worked that into the combat design by letting the combat table handle it. You simply get to modify the die roll if your attacker is within 6" of its target.
I also finally decided to have targets removed from play if they sustain 10+ friction points. Which means the armored car that pulled up in front of a platoon of soldiers would've immediately been removed from play due to opportunity fire. Players will have to get a little more creative than I was to avoid such a fate.
Regarding yesterday's attack on the car at the checkpoint...
That's a fair criticism. I think my vehicle rules will work ok for nearly every situation, and of course I wanted to keep it simple and fit it all on one page. The checkpoint scenario is a very unusual situation because it forces a player to move one unarmored vehicle within 1" of multiple armed enemies, knowing opportunity fire is going to happen first thing. That's simply not going to happen in most scenarios. So I think special scenario rules should be expected in such a case. And that would just read something like, "Attacks may not be conducted against the car."