The sales people at Best Buy should know... though it is a question that needs a proper answer, so they likely don't
You cannot use an aftermarket changer without a special connector. There
may be some type of aftermarket connector that can allow you to do this, but you'll need to track that down first as all CD changers use proprietary plugs.
You can get a Porsche changer on eBay for ~$200, though it's not very good.
As for the iPhone thing, you should get a better phone