For the Sanyo FT-818 the chassis ground and speaker ground wire leads are common... Also the FT-818 has NO balance control, just independent left and right volume level contriols. To test for Stereo output, insert a tape turn the Right volume level up with the Left volume level down, U should get sound output just from the right channel speaker. (Yellow wire to right speaker +, black wire to right speaker -). Next turn the Left volume level up with the right volume level down, U should get sound output just from the left channel. (Green wire to left speaker +, black wire to left speaker -).
The black wire should also be grounded to the car chassis..
Just my $0.02...