Don't forget that the connections for the rear speaker kit are back there. Could one of those plugs be for that add on kit?
There should be 3 plugs in there. C89 red 2 pin, C302 black 4 pin, and C304 brown 6 pin. C89 is definitely for the optional trunk light. C302 is for the rear speaker kit and C304 is for a passenger audio controller/ CB transmit.