Oldcarz,
There is a wiring picture for the routing of the tail light wire in the 1931 Chevrolet service news. I believe that you should only see the conduit section ,and it should duck up under the body as soon as possible. the wire itself runs over the top of the rear chassis rail ,and then along the inside of the chassis rail and body sill.
See the link to the online Chevrolet service news.
http://chevy.oldcarmanualproject.com/bulletins/31csn015.htmThere is also a clip listed in the 29-32 parts manual, page 94
Part number 355794 to suit AC AD AE C models, 'CLIP, Rear fender"
Some one may be able to provide a picture of the clip, I remember seeing a picture somewhere.