That's one of those things that most RPGs just aren't fit to deal with. Generally speaking, I'd say resolve the issue as you think best fits your game, using a "dramatic called shot". Trying to expand on that into an actual rule concerning called shots vs. armored vehicles might be a bad idea, as discussed in a whole lot of called shot threads in the history of these forums.
In reality, it would definitely be possible to render several sensor systems of an MBT inoperable with an AMR. Their covers, though armored, might not stand up to a close range straight angle shots with dedicated armor piercing rounds from weapons in calibers like 12.7x99mm, and at the very least there has to be an uncovered spot. The obvious problem with firing directly at the lenses of thermal cameras etc. is that the sensors have to be pointing in your direction for the shot to be possible, so make sure you evacuate your firing position ASAP...
An MBT of the 2070s would have loads of different sensor arrays, however, and several of them would be extremely difficult targets. For one thing, they might be tiny and camouflaged, and you could only find and identify them through your scope if you were intimately familiar with the vehicle in question. They might provide such a tiny target area that engaging them except at close ranges (a few hundred meters, tops) when the vehicle is static. And no matter what you do, the crew would still be able to use simple lense arrays to peer outside their vehicle without the aid of electronic sensors.
Disabling the crew weapons on top of the turret would be easy with any AMR. It is theoretically possible to puncture the small parts of the barrel of the main gun with a 12.7x99mm SLAP round, but because of the shape of target I would bet against it. I doubt putting a hole in the bore evacuator (the bulge about midway on the barrel) would disable the gun. In other words, I can't think of any realistic way to stop the big gun from operating with an AMR.
All in all, AMR vs. MBT would make for a horribly one-sided battle.
You can see what sort of ordnance has busted up MBTs (in this case, M1A1s) in reality
here at Wikipedia. There's one account of an M1A1 or A2 disabled (immobilized) with a HMG, lucky hits that set fire to fuel carried on the external equipment rack -- this would make the HMG crew responsible for it some of the luckiest Iraqi soldiers of the invasion, except that I doubt they survived to tell the tale