All vehicles are vulnerable from their rear facing, as it tends to be less armored (just like in real life). Therefore, AV-rated weapons gain a +4 DM bonus against ALL vehicles when targeting the rear facing of the vehicle.
AV vs. Impenetrability is a completely different process, and would stack when targeting vehicles with the Impenetrability SA.
Does that clear it up?
If sing, sang, and sung, sink, sank, and sunk, and drink, drank, and drunk, how is it that it isn't bring, brang, and brung, think, thank and thunk, and ding, dang, and dung?
Don't even get me started about bad, badder and baddest. Run, ran AND run...again? C'mon!