Most of the wiring going from the Junction block up to the Overhead will be bundled with the other wiring going to other items that are on the headliner. Things like lighted vanity mirrors. map lights, sunroof etc.
If the car has lighted vanity mirrors in the visors, I'd snag them...easy to install.
Pull the A pillar covers and other trim off around the headliner so you can access underneath it (Or above it). There's on Philips screw securing the overhead cover up by the windshield. Remove it and the cover pulls straight down held by two clips.
Cut the wiring under the headliner between the A & B pillars where it runs back to the rear reading lights if equipped. Otherwise just grab the whole harness that's under the headliner. The harness in a 300m or Concorde is the same so you can expand your search to those cars. Connections are the same whether OTIS or EVIC. Same with the cover mounting.