Steve
Most units in the army do use APU's on their tanks. The APU is usually mounted in the bustle rack in the turret's left side. If I'm not mistaken, the Tamiya kits that I have seen have the APU set up to mount to the tanks Right rear where the right tailight is mounted. This is where the army originally mounted the APU, but switched to the bustle rack because the APU would sometimes hit the ground when the tank went from flat ground to going up a hill. It may be helpfull if you can find some pictures from the unit and time period you are trying to create.
If you don't mount the APU there may be some pegs on the right rear that you should cut off. The pegs are there to hold the model APU to the tank.
Here is a link that shows an APU mounted in the bustle rack...
http://f3.grp.yahoofs.com/v1/8I1gP7jLrcDfWvCTkqEXGw2GPILBwaSsUSP3ghCYkk2y52Cfm5j4FPvIXEAHA0IY59Nxm7E6R5UR2yYRYcdXVK_OM2M/Model%20Abrams/Dirty%20Rear%20Load%20Plan.JPG
If the above doesn't work you can go to the below group, go to files, model abrams, and dirty rear load plan
http://groups.yahoo.com/group/armormod/