Why is my aux cable not working?

2002 mitsubishi eclipse with Kenwood KDC-MP245. When i play a CD all speakers work fine and it sounds great… BUT. When i plug in my aux cable for my iphone it only plays through one left side speaker. And i tried it on other systems and it plays fine… What's going on?

There's a short in the plug harness. The auxiliary cable has 2 places to contact, one for each speaker. I would take apart the plug and clean it and make sure the connections are not loose. If that doesn't work, replace the plug and check wires to stereo for continuity.