7 Pin Flat Plug Wiring. Verify the harness color codes at each tail light. There should be a common color for both the tail lights.
The thing is I have to go to the tail lights to make attachments. This plug uses flat blade pole terminal connectors instead of the common squeeze type that clearly have a tendency to bend over time. There should be a common color for both the tail lights.
The seven pin connector has pin assignments as follows.
This accounts for three of the colors in the wiring harness.
There quite a few types out there, and some types are more predominantly used depending on the state you live in. The flat plugs don't seem to be as durable, ground wire plug breaks off after a few years too often. The left and right turn signal lights should each have a different color.