Light green is indeed for the reverse lights. Brown is tail lights in general. Yellow is turn signal left side, dark green is turn signal right side. Black should be ground.
If yours is indeed wired yellow and black as you described (not just dirty discolored brown), then I would indeed replace the harness as it has been hacked up pretty good.
Digging up 6 year old post to give kudos. THANK YOU!!!!!!
1957 Thunderbird 289 1967 Firebird Base 461 1968 C-20 327
