The CD player you purchase must have a wireless remote with it however. Once the interface is installed, you can program it to correspond to any buttons on the CD player's remote by pressing the steering wheel controls.
As for an aftermarket unit that will control the factory changer...nope, ain't gonna happen.
Sorry, can't help you there. No such animal is made on the market in a package that will fit in a 1.5 DIN opening.
What I can offer for a suggestion as a replacement in dash CD changer is a Nakamichi MB-75. It starts at around $550 or so and has an auxiliary RCA input on the rear to accomodate an external audio source, such as a portable MP3 or such. ( http://www.nakamichiusa.com )
Other than that, I'm out of ideas for MP3 and an in dash changer.