That is the resistor mod to eliminate the EGT sensor. It is usually used when installing an aftermarket uppipe or when the EGT sensor has failed. It has nothing to do with the rear 02 sensor or the P0420 cat inefficiency code.
Rather than experiment with resistors on the rear 02 that may or may not work, you can get an inexpensive kit to install that will eliminate the CEL. Do a search here and on the NASIOC site for "MIL Eliminator", "CEL Fix", "P0420", etc. and you should find what you need.

Edit- actually, re-reading your post the only thing you mention having installed is the uppipe. If you still have your original downpipe w/cat it should be sufficient to keep the rear 02 happy. You won't normally get a P0420 from just an uppipe.
What did you do with your EGT sensor that was in the original uppipe? Did you reinstall it in the new one or just disconnect it? If you removed it that is why you have a CEL, if you reinstalled it it may have failed.
I suggest you get the code read to be sure of what it is. There are several of us in the area that can scan it for you, or just stop by an Autozone and they will do it free.
If it is the EGT sensor (or lack thereof) causing the code then yes, the 2.2K resistor mod will take care of it. The resistor can be installed in the connector that goes to the sensor probe, located under the cover by the passenger side strut tower by the boost control solenoid. The plug that goes to the probe itself is disconnected, assuming it hasn't been already, and the resistor leads can be inserted directly into the pins of the mating connector.

 

black electric tape...just cover it up...