Such a device probably exists, the trick is finding it. Here is an example of one such device. However, it may not be the right device for your particular car. Have a look at PAC, www.tune-town.com, www.rcainput.com and other similar places.

Another thing to worry about, some cars with a Bose amplifier, the amp only powers some of the speakers and the other speakers are powered by the head unit. Do some googling and see what you can come up with regarding specs on your particular model.