I've done the Becker conversion. $100.00 plus freight. This is by far the cheapest option.
You get a nice long cable with a good receptacle. I just mounted mine on the passenger side tunnel. Just remember that you must have a tape in while you are using the aux input.

Good people at Becker. A few years ago I had an amp go out in my 300E. I was just north of Upper Saddle River on business. Gave them a call and they said sure stop by.
Ed Ebel and a technician came out to the car, did a diagnosis and replaced the amp in no time at all.

Steve