Disclaimer - I am not the smartest guy here. I am no longer in parts management. However, I do have a few suggestions / ramblings.
State codes are good for this. However, when I first ran into this the field "SC" was not in our RPG dictionary. I had to have ADP insert it or "build the dict item." Therefore, if you cannot find the field in your RPG they CAN add it for you. You may have to run the report a few times, modify it or pull parts of it into excel and sort it the same as the KPI report.
You can go out on the RIM website and simulate a return using a date months from now. This will give you what the RIM system will actually allow you to return. THERE WILL BE ERRORS! It is entirely up to you (dealer PM) to find them. Once you can prove the mistake, RIM will take the part back. YOU WILL HAVE TO PROVE THE MISTAKE.
Alternative, for later assistance. We input a bin2 location on every RIM part number. For example, now we are using RIM16. This tells us the part was ordered by RIM during calendar year 2016. As parts reorder we adjust bin2 as necessary to reflect proper year. The first couple months of the year we will change lots of parts from RIM15 to RIM16. This helps when we look at MNS>15 and see a RIM14 part number.
I know some PMs create a 'group' of sources annually and do much the same thing with them that we do with bin locations. Each year a new group of sources is created that mirrors the GM sources. For example if GM uses 100 - 110, then this year's parts are in 200 - 210, next years 300 - 310 or whatever numbers you want. When 15 - 18 months pass those sources should be empty.
Granted this is a lot of work to keep straight - but when has parts been easy.
If no internal (dealership) RIM separation has been used, then you will have some time to figure it out using RPG / Enhanced Report Generator / Excel and KPI. If this is the case I would probably start by pulling each state code into an excel sheet. Cut and paste all the excel files to create a single file. Or simply create a single excel sheet. Sort the same as the KPI and compare them. Not easy but work it a little each day.
Good Luck