Just looked in the MB electrical manual. Circuits don't get any simpler. Power starts at fuse F14 to pin #1 at connector C146 to back up light switch and back to pin #2 of C146 then to pin #6 at connector C104 in the trunk and then to pin #4 at both tail light assemblys. Take the fuse out and look at it even if it looks good. F14 also supplies the horn.
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