S13 SR20 Harness into 91' S13 KA24DE Chassis
 

Can anyone provide wiring diagram/pin out for gray connector behind battery?
Honestly, I need to know wire for wire are they supposed to match? I know there are some wires with respect to color that are not on the KA harness. Any help would be greatly appreciated.

This will be a good start for you. Should have everything you need to find that pinout. The harness layout is around pg. 90 for the engine bay, and then from there you can figure out what pinout/connector you need to look for.

Hope this helps!