Hi,
This red wire for the radio is AFAIK present in all the wiring harnesses, at least on the 280's, possibly also on the 230 and 250's.
If the car came with a factory radio(Becker) option it was used in conjunction with the bracket and fusebox on the right inner fender.
In other cases it was taped down and hidden in the harness, but still present to be retrieved when needed.
Hans