Bob,
In a stereo shop, they actually connect it to a known working tuner, speakers, and power source. That is how it gets tested.
I'm sorry. I wish I had an easier answer, but that is the only way I know how it's done. One of my best friends owns a stereo shop and he has checked many for me that way.
At a car dealership, they would test it by installing a working one to see if everything starts working.
Let me know if that helps answer your question or if I can help in any way.
Take care,
Joe
Thursday, June 8th, 2023 AT 8:01 PM