I was working on a 211 estate today and I noticed that the new Audio 20 units with bluetooth have two ariel amps , can any one tell me what the function of the extra amp on the drivers side is
You mean the FM antenna amplifiers at both sides of the car? An S211 wagon would have a traditional 2-branch diversity receiver radio, hence two antennas and antenna amplifiers.
A sedan would have multiple FM antennas but since all are located close to each other at the rear screen, a single antenna amplifier can pick up the the best reception path. This is done under the control of the head unit ZF-signal (sort of feed-back about the signal quality the tuner would see).