I'll disagree with Britton 100%. Particularly in a 280 se coupe...
My opinion is that anything in the dash that isn't Becker looks like Scheiße and that there are other options.

1. If you just want a bluetooth boom box, I have one from Sony, and it sounds great. It should be trivial to wire a power supply to it from somewhere else in the car. If you are concerned about theft, stick it in the trunk.

2. Someone in Orange County has a Becker setup with custom bluetooth installation - I might have the link saved in ebay... Becker Radio Europa II with Bluetooth Music Pandora Streaming iPhone Andrroid | eBay

3. Beckers with separate amplifiers - someone else is selling a cable setup to take the becker head unit cable and convert it to run a standard after-market amp. That gets you power for "big" speakers. Most Beckers have a plug in back that you can buy a cable/box for that allows you to have a 1/8" input jack somewhere in your car. Not BT but does provide a way to have your player play through your Becker.

There are other options besides putting in an aftermarket head unit, or even worse, a "Retrosound" headunit...