First, you do this at your own risk. If you need to install a new Rear Carriage Bushing, I think the odds of you screwing something up are infinitely greater if you follow the instructions in the z3100 Service Manual (take the whole printer apart) than doing what I outline below (only remove the Right Cover), but, bottom line, they tell you to do it their way. I think my way is better, but don't blame me if you screw it up.
If you replace your belt on the HP z3100 / HP z3200 it is EASY to break or lose the Rear Carriage Bushing. It is also VERY EASY to miss this problem until AFTER you have replaced your belt and reassembled your printer and find that it is dragging the heads or the carriage on your paper/canvas, streaking/smearing ink, and making some loud banging noises when the Carriage travels to the right. So when you replace your belt, double and triple check that your Rear Carriage Bushing is on AND that it isn't cracked (it is extraordinarily flimsy plastic) and you won't have to do what I did.
BUT if you need to replace or install the Rear Carriage Bushing on the z3100 / z3200, the manual tells you to essentially disassemble the ENTIRE printer just like you have to do to replace the belt. YOU DO NOT HAVE TO DO THIS (if you're careful.) Just remove the Right Cover. When you pull the Carriage all the way to the right, (use a bright light shining in there so you can see), you will see the hole where the far end of the Bushing goes and the hole where the near part of the Bushing goes (look at the diagram in the Service Manual). I just used a pair of needle nose pliers to hold the bushing so I didn't lose it, (don't let go!) and using a tiny 4 inch screwdriver (tiny like you could remove the back of a watch with it), was able to gently force the far end of the bushing into the far hole. Then CONTINUING TO HOLD ON TO THE BUSHING WITH YOUR PLIERS, push or pull back that fleshy colored piece of rubber thingy in the way, and you can easily apply tension to install the near end of the bushing into the corresponding hole. If you bend the fleshy rubber thingy a bit, use your pliers to bend it back like it was. This saved me about 8 hours of headache NOT having to tear down and rebuild the entire printer AGAIN, and eliminated the likelihood that I could just as easily lose/break the bushing again when having to put the Carriage back on!
I did this on the HP z3100. I have not done it on the z3200, but my understanding is the printers are exactly the same in construction.