My brother was the previous owner of my truck, he'd removed the stock radio and had an after-market one put in. When he bought a different truck, he removed the stereo system. When I got the truck, I went to the junkyard, got a stock radio and put it in myself. The radio itself works but only one of the six speakers are working ( front speaker in the passenger door). The truck came with an Infiniti sound system and still has the original speakers.

I asked my brother if all speakers were working at the time he removed the aftermarket stereo and he replied that they had in fact worked. I took him at his word since he has no reason to be dishonest about it.

Any suggestions as to what the issue may be?

 

Take the one working speaker and try it in all the other locations to determine if the issue is the speakers themselves or the used head unit you purchased.

 

There is a simple way to test the speakers if you have a voltmeter, you can test for continuity in the speaker. That way you don't have install the working speaker over and over.