So can you use attack ground on your tank that is front facing a target tank (similar to that sherman to the left of the tiger) to get rear armor shot or does your tank automatically target for the rear of the other tank if the shot is possible?
This game has shot collision, so even if you target the rear, the shot will hit the front first if the other tank is facing directly towards your tank.
However, if the other tank has his side exposed, it is possible to manually aim with attack ground the rear portion of that tank to get a rear hit. However, this is probably not a good idea, since tanks have a scatter so they aren't that accurate with attack ground.
And no, this game is not smart enough to target rear armor on its own.