I too would prefer to maintain the look of the factory deck, but it would be nice to have bluetooth ability. I found this, but I have yet to try it out. But this is what I plan on doing. Take a look:
AUX-BKR Auxiliay input Jack for Select Becker Radios with CD Changer :: Auxiliary Input Adapters
or
3.5-BKRCD Auxiliay input Adapter for Becker Radios with CD Changer :: Auxiliary Input Adapters
Plus you'll want this:
BRRT Becker Radio Removal Tools :: Accessories
I intend on getting the one that adds the AUX female plug that I'll install somewhere in the cockpit. This particular one requires that the CD changer remains connected and operational. The internal software in the stereo deck already have the previsions for an AUX input.
It's not an ideal BT connection because it will only be BT connected when in AUX mode, but for purposes of streaming music, it'll work just fine. On the other hand, say you're listening to the radio and a call comes in, it will not be BT connected (only when you're in AUX mode).
This is how I understand the system to work, but again, I have yet to actual do the install. If you like this and beat me to it, please do tell!!
Good luck,