PIE produces an interface adapter to allow the use of an aftermarket Panasonic CD changer with the factory head-unit in 1998+ model year Audis (older model year owners are still SOL). They also now support the Bose (& other) radios in new VWs.

I believe you have to buy these adapters from retail car stereo/audio stores (can't buy factory-direct) but for details visit their web-site by clicking here.

The application charts can be found by clicking here.