Left front and rear car speakers don't work when using aux port?

So I just got a 1994 Ford Mustang GT, all front and rear speakers work when I use the radio/CD, but when I plug in my aux and play my iPod, only the right side front and rear speakers work… What could be the problem?

Is could be your aux cord I use an fm transmitter and the wire that went from the transmitter to my phone started going out but it was because the cord was wearing out just try a new aux cord I'm sure it will work and nice car I love them Thanks, I'll try it tomorrow when I have time

Thanks, I'll try it tomorrow when I have time