etrailer is probably the best place to start. They have everything you need, and are very helpful. To start you can enter your vehicle make/model/year, and the website will bring up the correct parts for you. For example, when I ordered the break system for my '88 Bronco, this is the wiring kit I ordered:
https://www.etrailer.com/faq-etbc7.aspx
It's easy enough to do yourself - a few hours, the correct tools, and you'll have your new-to-you vehicle wired for your TM.