Louie,
FWIW, I agree with Mike on all counts including his two separate recommendations on how to bring the car back closer to as it left the factory. Let me throw out a little history as to what may have lead your dad to have the changes made to the car....
In the 1960's, most cars had little to offer in the area of sound systems. In fact, the only decent sounding system that a person might have owned at the time sat in their living room and looked like the one in the photo below. In our house, it was simply known as "the stereo". Any other unit around the home was likely monaural and may have only pulled in the AM band as that's how most popular stations broadcast. Most of these units (including the portable transistor version) were simply referred to as "radio's". In the early '70's however, smaller and more home systems started becoming readily available (many from Japan) and the listener had a myriad of new options for improved sound. Also, too, FM radio stations proliferated and most of these broadcast in stereo.
From here, consumers began demanding for a better sound quality out of their automobiles. Up until the '60's, we were more interested in cars that were simply cars; in other words the "sound system" as it was, being an after thought. Think of this, 113's were THREE TIMES more expensive than a typical Chevrolet of the time but yet a radio was not even a standard item, it had to be ordered as an option. Even more so than US drivers, Germans were more about driving than about creature comforts (MB didn't even offer cup-holders in cars until about 10 years after everyone else).
So your dad likely had a dilemma on his hands...a nice Mercedes with a lousy sound system. Rather than doing what many other owners did at the time like add an aftermarket unit from Pioneer or Sony, he ordered up the best system that MB had to offer, a Becker model designed for the newer SL model of the day (the 107). As Mike suggests though, whomever handled the work for him butchered the installation of at least the rear panel speakers.
While your baby is off the road for the winter, you have the chance to consider next steps on your sound system. I would guess that most members here would recommend that you try to bring things back to as "factory" as possible. I personally feel that this portion of your plan does not have to necessarily be carried out by an MB Master Tech ....I do agree that any mechanical work should be... but it should be carried out by a reputable shop which Mann very well could be.
Other things to consider would be finding or ordering your car's "data card" from the MB Classic Center in Irvine, CA (See
http://www.sl113.org/wiki/DataCard/DataCards and look for items 510 - 519
http://www.sl113.org/wiki/DataCard/OptionCodes ). This and/or your father's original paperwork will show you exactly which radio the car was built with. From what we can see here, it was not the unit that is in the car now.
Good Luck and keep us posted!
Kevin
