I think there is an access cover, curbside, just about fore of the axel, in the belly area.
All your umbilical cord wires go into that, from there they then go to their respective locations (tail lights and marker lights).
If you are just trying to diagnose a problem in say one of your marker lights, or maybe a taillight, you can start there and check for continuity.
If you want to rewire all the lights, then yes, you would have to pull the interior panels to do this, as the wiring runs between inside and outside shell.
Hope this helps,
Thanks in advance!