car stereo issues

Im really not sure where to go with this...

I have been listening to music in the car since I got it, usually on my iphone through an auxillary cable. Plug it into my car and put it in my iphone, music plays. But...over time....Things have been going south. And specifically, it IS getting weird to hear music coming from the left speakers and not the right, but a perfect mix/balance when I listen to the radio. It was fine when I got the car but over the past few months Ive been hearing little to nothing on the right side.

I have dropped my iphone plenty of times, and that cable I have used for atleast a couple of years. But I also tried it with my sisters iPhone and it didnt work. Plugged the cord in my moms car and it worked fine with my iphone.

Yes, Im making sure that it is jammed in all the way in both ends, and that I didnt have the car's stereo "balance" feature on the left side.

Is this a car issue or a cord or iphone issue? I called Radio Shack and a guy told me that replacing the cord wont do anything...and his bluntness wasnt very helpful. Was wondering if anyone can help?

Again sorry...didnt know where to go. I figured the aux cable to be an iphone accessory >.<

I think it might be the cable.
My music plays fine in my moms car when I plug in the iphone, as does my sisters phone.

When we plug our iphones in my car the left speaker plays like it should, and the right nearly gives out. Guess Ill buy a new aux cable at radio shack or best buy?

Should that not fix the problem, do I contact the place where I bought my car for service or should I go test another iphone/ipod out

How do you replace that?
Its built in to my car I believe. I cant get a good look at it but Im pretty sure its built in.

Thank you!!

And should I need to replace the head unit, who would I go to? my dealership?