I have a boy that has a 1993 Mazda B2600 that is giving him issues with the stereo. It was playing then he did something with the amp and it just quit.
Checked fuses, and even put a new system in it wont play but hook it up to the Dodge it plays. This is not a factory system either. Hair is getting grayer by the minute. Any ideas to help him get this working? Please message me back as soon as possible...

 

As with all electrical problems, its best to start diagnosing from one end and work toward the other until the problem is found.
Since you say the head unit plays when connected to another vehicle, it's liable to be downstream from that. Or if you are talking about the radio itself not pulling in stations, check into the antenna being the problem.