If both speakers work in one port but not the other, then it needs pulling apart!.. I believe the amp is a TDA8510 or there abouts.. Just check the connections from the faulty RCA jack to the amp board... Other than that replace the TDA... Also check the amp input just in case its on the pre-amp side..
hey did yu check stereo pin connections ?
primarily check the speakers with a dc source / battery
see any of amplifier components bulged out , if it happened replace with a new one ,be careful while dealing with ics and transistor + diodes