Someone had posted on this forum that because the 230sl radio opening is bigger, they reached the speedo from there. I don't know personally, but if that is right it will make things easier.
Assuming that it is, once you have disconnected the cable at the trans, and at the back of the speedometer, you want to pull it thru the dash into the interior of the car. Tie a string onto the end of the cable to which you will attach the new cable and pull it thru the dash and over to the trans.
There are several good step by steps. Use the search tool.
Gus
280sl