I've had the later generation Infinity factory amp apart
, there's not provision in the device for individual fusing.. in fact the IC's drive the external connector pins directly, you can see the big fat traces on the PCB right here
(they go under the rail caps, but connect directly to the drive IC's) - so if you have a dead channel, the amp's dead. See if someone's selling/giving away theirs, or start shopping for an aftermarket replacement
Note: since it's a rear channel output not working, you may also be able to simply bypass the Infinity amp and drive the rear speakers directly with the head unit. I'd suggest doing this for both left and right at the same time and making sure you get polarity correct (wrong polarity won't cause a failure, just bad sound) - it's a cheap-and-dirty way to do it, but I'm not above doing cheap things when the cost isn't worth the results.