iPhone 3GS not working with any docks, ideas?

I have an iPhone 3GS that one day (about a month or two ago) just stopped working with docks in terms of playing music. It can still charge via docks and I can still connect it to my computer and iTunes for syncing, but it just will not play music through the dock connector. I have since upgraded to an iPhone 5, which is why I have not gotten around to try and fix this issue yet, but I would like to see if there is a solution as I would enjoy using my 3GS as a permanent iPod for my car (via the car's dock connector obviously). I have tried cleaning the port on the phone with a slightly wet cloth and toothpick, and also some alcohol, but no luck. Any ideas or tips?