A yellow RCA typically represents "video". The black RCA could possibly be the sub out. That would leave the red and white connections for "right" and "left" respectively.

Otherwise, if I'm in left field there, I would guess the yellow and black would be the rear channel outputs, so that the HU fader can control front and rear.

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car