unable to get reverse.. w123 240td auto

There's a car for sale with following sympton ....'Not starting in PARK, but starting in other gears. Then I could get reverse, in between park and reverse, then sometimes not at all. Fault is intermittent, but bloody annoying when you cannot get reverse. I was told it may be a faulty parking switch in the gearbox.'

Any one familiar with these transmissions have any ideas? Is reverse engaged mechanically or is there some eletrical inhibit/conection.