lol, the local stereo shop, unless they have an old timer working there those guys are going to think you're the NSA or something and not have a clue what you are asking them to do.

IIRC, and it has been many decades so CRS comes into play here, but red is switched 12v, black is ground. Car antenna into the box, it's antenna lead into the radio. You tune the car radio to a certain AM frequency, usually they have several in case a local AM station is near one of the frequencies. The converter will take the FM station it is tuned to, convert the audio to an AM signal, the one you tune your AM radio to, and your AM radio decodes it back into audio for your speakers to play.

The only reason they appeared to work decently was due to the general state of crappy audio back then.