Baofeng UV5R Repeater
Thanks to this article, I successfully created a cable that lets two UV5R’s work as a repeater:
https://survivalcomms.wordpress.com/2016/05/22/setting-up-2-radios-as-a-repeater/comment-page-1/
The diagram below was the key.

I cut the audio cable from the dollar tree in half. Plugged one into the adapter and used a meter to determine which wire went to what part of the plug. Soldering the appropriate wires together did the trick. (Note that the wires have a coating that “burns” off during the soldering process.