I'm looking to do the same on my 1982 300D (auto transmission.) To change the shift knob, you have to remove the wood console, unscrew the locknut, and replace the shifter linkage with a new piece from the dealer. Next, you attach your new shift knob to the new shifter linkage. A friend of mine did it another way, and I'm planning to do it this way too. Use a hedge clipper (or another tool of choice) to chop off as much rubber from the shift knob as you can. Then it'll be pretty easy to remove whatever remaining rubber there is on the metal shaft. Now you're ready to install your new shift knob!