‘68 Dart, One tail light not working.

Do you have the shop manual and parts books? Pretty sure www.mymopar.com has both for '68. I have paper original and CD for 68 so haven't looked but pretty sure mymopar has them.

Yellow insulation was used for dome light return, map light, etc. Orange was typically used for instrument lights including optional tach, shift indicator, etc.
Since there is a delay unit, there could be some other optional or less typical harness connections.