1985 Mercedes 300CD Reverse Gone Out all of a sudden-like

Hi, I have been searching this newsgroup for a couple of days now looking for any hints as to what may be causing my friend's 300CD Reverse gear
to have stopped working all of a sudden. She claims that a shop did some work (oil change) and the next day reverse stopped working. Also when she left the shop the radio and power windows on one side no longer worked. I found the loose connectors and re-connected the plugs for the fader and the windows, but have been unable to figure out the tranny thing. I found one mention of a belt adjustment (internal to the trans) in the group. Can anyone else shed some light on this? I am trying to help her not get ripped-off on this repair, so I would mainly like to know if I could do the repair, or if a trans shop should do it. Thanks, Andy
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