Here is an instruction on how to do the separation. Please note that it explains how to replace
the reverse lights with the turn lights.
However, if you want to add the indicator function to the reverse lights without disabling them, you will need some additional instructions. I didn't do it myself, but being an EEE major, I am sure it would work just fine. I will post them in a couple of days because right now I am struggling with the midterms.