I have a barely used (was on the car 2 weeks before it was totaled) car-to-trailer wiring harness by eTrailer.
The wires are a bit shorter than it came with as I clipped them to remove it, otherwise it's good to go, including instructions. You may need some quick-splice wiring connectors for the easiest installation.
"Adapts import and domestic vehicles with separate turn and stop lights (3-Wire System) to standard trailer taillight wiring (2-Wire System)."
More details here or below. I paid a discounted $35. plus shipping.

$10. and it's yours, and I'll donate to proceeds to charity, either the Karen Hornbecker Fund or RRW Airfence Fund.
Pick up at the track on a race weekend or I can mail it for probably $3 in a padded envelope..
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Draw-Tite Wiring - 119175KIT
Tow Ready Circuit Protected Taillight Converter with 4-Flat End by Draw-Tite, Hidden Hitch and Reese

Adapts import and domestic vehicles with separate turn and stop lights (3-Wire System) to standard trailer taillight wiring (2-Wire System). In a three wire system, a separate wire is used to carry each of the stop, turn, and running light functions to the taillight assembly. On most trailers, the stop and turn signal functions are sent through the same wire.

Features:

  • Short circuit protection safeguards itself and the tow vehicle against harmful electrical shorts and mis-wire situations
  • Circuit automatically resets once the module cools
  • Adapts a 3 wire system on the towing vehicle to a 2 wire system on the trailer
  • Will not work on vehicles with LED taillights
  • Works with most vehicle with multiplex wiring system for lighting
  • Includes circuit tester and (5) Quick splices to aid in installation.
    • Also includes 4-pole trailer connector cover


Specs:

  • Wire pigtails measure 18" long
  • 4-pole trailer connector is 60" long from converter box