As you probably know, those flat wire harnesses in the wagon are not reproduced. I would think your best bet would be to run a new separate wire for the broken one until you can find a decent used harness. Try True-connections.com. They have some used parts and may be able to hook you up.
After pulling the carpet back, you'll probably be able to see where the ribbon cable is cracked/broken. By carefully scraping away the plastic insulation and exposing/cleaning the copper, you could probably make a "solder bridge" across the crack with a medium wattage iron and ROSIN core solder. Re-insulate it with a few wraps of tape or that aerosol "rubber in a can" spray. An idea might be to put a length of a wooden paint stir stick on the back side to keep it from flexing at this point again during the tape wrapping.
It's not a short, a short would blow the fuse.
It's a broken wire, should be easy to fix like DG said.
Get a test light with a sharp point and probe the wire with the park/tail lights turned on until you find a place where you have power and and a place where you don't have power and you have found the break.
I'm sorta with Jim on this one. If the tail lights are acting up I'd snake a new wire under the carpet and fix the problem. It's a brown wire to the tail lights.
Suspect sometime in the future another line will start acting up because the flex sounds like it bent and is damaged in that area under the foot. I wouldn't disturb it unless I was changing it out because guessing the other traces will go if you play with it.
After that keep and eye out for a replacement.
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