Hey guys,
While trying to track down a wiring harness for my 89 W-150 I wrote to Mopar and asked their assistance. Here's the response:

Thank you for contacting the Dodge Customer Assistance Center. I can certainly understand your desire to restore your vehicle using original (or as close to as original) parts as possible. I want to get you in touch with some resources that could be able to help you out.
We have a consolidated resource that we launched in 2016, the Mopar Restoration Portal. Located at https://eur02.safelinks.protection.o...%3D&reserved=0
, this site provides enthusiasts like yourself Timothy information and access to more than 45 approved Mopar restoration parts licensees, and more than 9,000 restoration parts and accessories can be sourced.
It is my hope that this will lead to you being able to locate the wiringharness you are looking for.
Thank you again for your email Timothy
Sincerely,
Alex
Customer Service Representative
Dodge Customer Assistance Center